You can certainly use those low CRs... but you just need to be okay with the idea that if you want to do substantial damage to the characters, you need to have a larger number of them.
Take a look at the monsters' attack bonuses and damage numbers. They'll probably be in the +3 to +5 range (meaning you'll need to roll probably 12s and up to hit the PCs) and do only usually maybe 5 to 9 damage per attack when you hit. So when you compare it to the HP your two fighters and the warlock have... you can kinda calculate just how many monster attacks you need to hit each round to knock these PCs down to like half HP or even lower (if that's what you're hoping for.) Usually, it'll mean you'll need probably 2 to 4 times the number of CR 1 or CR 1/2 enemies to start doing any damage to them (especially if they also have abilities that mitigate the damage beyond just their AC.) I personally have found that usually you'll need more enemies swarming if they don't have Pack Tactics as an ability (giving them all Advantage when they attack), and less if you do.
So for instance... with 3 PCs, if you swarm them with a pack of 10 kobolds so there are 4 on each fighter and 2 on the warlock, they'll do a good number on them usually before they are able to take out the kobolds (unless of course the party is able to get the jump on them, in which case they might remove half to 2/3rds the threat in the surprise round before the kobolds ever get to respond.) But because the fighters and the warlock are more individual enemy based, you don't have to worry as much of seeing the entire swarm of enemies taken out in one fell swoop (as you would if you had 1 or more PCs with large AoE attacks.)
The other way to handle it is to have the low CR monsters set up so that they could come in waves. 5 goblins in the first area with at least one or two in the back ready to run if the ones in front get overwhelmed. The one that runs away calls for reinforcements so that the next group of 5 arrive after the group has taken out the first threat. After that, you again can see how well or poorly the PCs have been to remove the threat before deciding whether another goblin goes rushing off for even more reinforcements. It's a good way to have larger fights available to you as a possibility without completely overrunning the PCs from the very beginning.
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