uh, boss - a thought occurs. the earliest point RAW at which you can have 3 feats while multiclassing with exactly 3 levels in another class is level 15.
Do you mean level 12? Go 3 levels of each to qualify for the base feat (total of 6 levels), but no feat slot yet so you can't take the the feat. Gain 1 level in either class (let's say Berserker so you're 4/3) gets you an option for a feat that you use on the Runespell Rager Feat. Gain 1 level in the other class (so you're now 4/4) gets you another option for a feat (choosing Rune-Linked Mind and Body). Then gain another 4 levels in one of the classes (let's say Warlock, so you're now 4/8 Berserker/Warlock) get's you an another feat option where you choose the last feat (Pact and Rage Rescribed).
Here's a sample for a berserker/warlock synergy feat chain that I've been tinkering with:
That said in regards to the feat chain some observations (not necessarily critiques, but things that stand out to me) with explanations on why I'm pointing them out:
From
Runespell Rager Feat: "You may use your Constitution modifier in place of your spellcasting ability modifier for attack and damage rolls; for damage this only applies to your Eldritch Blast attacks if you have the Agonizing Blast invocation" - I would be very wary of moving spellcasting to Con, especially in this way.
Explanation: This makes the class combo more MAD since you have to level up Str, Con, and Cha to try and keep Melees Attacks, Magic Attacks, and Regular Spells all equally effective. I know in A5E this is less a factor than O5E, but it's still a factor and will affect some players. Also making Con a spellcasting ability MAY (I emphasize may here) create a bit of an imbalance with casting. You now have a character in melee with prof in Con saves (assuming Berserker first) and a high con mod so concentration checks may become moot. I'm not 100% sure that it's a big issue, but I've always been wary of it.
From
Rune-linked Mind and Body Feat: "In addition, if you cast a warlock spell while raging you may spend one exertion to increase your warlock spell DC or your warlock spell attack modifier by an amount equal to the rage hit points you gain every turn." - This is insane to me.
Explanation: Let's look at a few numbers here. Let's assume you're playing a character that focuses Berserker while taking this feat chain. You would qualify for this as early as 8th level (see my response to
@ReadyButNot above) when you're a 4th Berserker/4th Warlock. At that level you're sacking an exertion (not a huge resource) to gain a +2 Spell Attack and DC. A +2 is significant in 5E. If you max out the Berserker side you're looking at a bonus of +3 gained at 13th level (9th Berserker/4th Warlock) and +4 gained at 20th level (16th Berserker/4th Warlock). That's not a fast progression, but in a system based on bounded accuracy those bonuses are huge. Is this to make up for the fact that caster may not be able to increase their Cha due to picking feasts over ASIs? If so it works for a character that emphasizes Str, but can get unwieldy if the character emphasizes Cha.
Possible Change: Have you thought of allowing the character to cast through an attack? Maybe something like this "In addition, if you cast a warlock spell that targets a creature or has an area of effect you may spend one exertion to cast the spell through a melee attack. You make a single melee attack as normal and the target, regardless of if the attack hits or misses, is the target of the spell, if the spell targets a creature, or becomes the origin of the spell, if the spell is an area of effect."
From
Pact and Rage Rescribed feat: "You may enter a berserker rage by expending a number of warlock spell points. You must expend a number of spell points equal to your proficiency bonus or the cost to cast the highest spell level available to you through your warlock levels, whichever is higher." - I have to agree with ReadyButNot that having an ability that costs more with no added benefit just because you're higher level doesn't make any sense to me.
Mystic Maneuver: I also agree with ReadyButNot that this is overpowered.
Overall I love it and hope to see more.