OK, I'm sure this will come across as an attempt to "win."
That is not the case.
This is just to explain my thinking. Why I read the rule in this way. If I'm wrong, I'm OK with that. This discussion is one that makes me think (and research) the rules, which is really the sort of thing I'm looking for in these discussions. To make me think. And then decide how we'll adjudicate it in our game.
I've also asked Jeremy Crawford directly, so we'll see if he replies. Again, if he says I'm reading the rules wrong, that's fine. It seems that there is some confusion (meaning different people read the rules differently), and it will probably benefit others to have clarification from him.
Also, I've truncated some of the original text from the PHB and Sage Advice. It's not to pick and choose, just to save space. I think I pulled the relevant text, but if I missed something, just let me know.
Anyway, I think they rules say you can cast a spell as a bonus action (such as misty step), then a cantrip (fire bolt) as an action, and then a spell as a reaction (counterspell).
This wasn't always my thought, but based off of several answers in Sage Advice, I now believe this is the case. Why?
To start with the simple, here are the Sage Advice questions:
Is there a limit to the number of spells you can cast on your turn?
"No, no set amount, but if you cast a spell as a bonus action, you can cast another spell with your action, but it must be a cantrip."
Can you also cast a reaction spell on your turn?
"You sure can!"
That sequence is really enough for me.
So let's go back and see if that makes sense:
PHB 202: "A spell cast with a bonus action is especially swift. You must use a bonus action on your turn to cast the spell, provided that you haven't already taken a bonus action this turn. You can't cast another spell during the same turn, except for a cantrip with a casting time of 1 action."
OK, so what's a turn?
PHB pg 189 under the heading "Your Turn: On your turn you can move a distance up to your speed and take one action.
There is a subheading called: "Bonus Actions" on the same page where it says, "...you can only take one bonus action on your turn..." among other things.
Reactions, on the other hand, are in a separate section, that is, they are not a subheading of "Your Turn." Reactions say, "...which can occur on your turn or on someone else's."
But since reactions are not a subsection of Your Turn, my assumption is that they aren't a part of your turn.
So we'll check Sage Advice. First, this is what opened my eyes about how a reaction works a bit differently, and how important the term Turn is, and it was one of many that also reiterated that the terms they use are used for a specific reason.
Can a rogue use Sneak Attack more than once per round? "Yes, but no more than once per turn. In combat, a round comprises the turns of the combatants (see the Player’s Handbook, p. 189). Many features in the game, such
as Extra Attack, specify that they work only on your turn. The Sneak Attack description specifes that you can use the feature once per turn, but it’s not limited to your turn. The feature also doesn’t limit the number of times you can use it in a round.
This rule is relevant because you sometimes get a chance to use Sneak Attack on someone else’s turn. The most common way for this to happen is when a foe provokes an opportunity attack from you. If the requirements for Sneak Attack are met, your opportunity attack can benefit from that feature. Similarly, a fighter could use Commander’s Strike to grant you an attack on the ghter’s turn, and if the attack qualites, it can use Sneak Attack. Both of those options rely on your reaction, so you could do only one of them in a round.
Because of getting only one reaction per round, you’re unlikely to use Sneak Attack more than twice in a round: once with your action and once with your reaction."
This was an "aha!" moment for me. It hadn't occurred to me that you could get a second Sneak Attack due to your reaction. This post from some time ago was the one that identified that a "turn" was different than a "round."
Then there are the specific questions about casting time:
Is there a limit on the number of spells you can cast on your turn? "There’s no rule that says you can cast only X number of spells on your turn, but there are some practical limits. The main limiting factor is your action. Most spells require an action to cast, and unless you use a feature like the fighter’s Action Surge, you have only one action on your turn.
If you cast a spell, such as healing word, with a bonus action, you can cast another spell with your action, but that other spell must be a cantrip. Keep in mind that this particular limit is specific to spells that use a bonus action. For instance, if you cast a second spell using Action Surge, you aren’t limited to casting a cantrip with it."
So this question is specifically about casting on your turn. In addition, it only talks about actions and bonus actions. Again, based on the PHB definition and the clarification on Sneak Attack in Sage Advice, this makes sense, since a reaction is not part of your turn. At least the way I understand it.
The second question is about bonus actions, but doesn't apply directly to this question.
The third question is:
Can you also cast a reaction spell on your turn? "You sure can! Here’s a common way for it to happen: Cornelius the wizard is casting fireball on his turn, and his foe casts counterspell on him. Cornelius has counterspell prepared, so he uses his reaction to cast it and break his foe’s counterspell before it can stop fireball."
Note how that third question is worded - "Can you also...?" Throughout the Sage Advice document, there are several instances where one question leads into the next. That is, each question sequentially is expanding upon similar questions in the same section, if there are similar questions. Now I realize the example doesn't include a bonus action. Personally I see that as an oversight. They could have clarified it fully with the answer to that question, but they didn't. So I can see how the rule is still somewhat ambiguous.
So back to the start which is how I read that section in Sage Advice:
Is there a limit to the number of spells you can cast on your turn?
"No, no set amount, but if you cast a spell as a bonus action, you can cast another spell with your action, but it must be a cantrip."
Can you also cast a reaction spell on your turn?
"You sure can!"