3 Flyrants are really, really a pain in the rear end for CSM, given their lack of good anti-air. That problem is only made worse in the Daemonkin book: a Vector Strike and Lash of Khorne ain't much to speak of, for some damned expensive minis...
Flyrants can't hurt AV13+, like they say, but I'm not seeing any bunkers, Soul Grinders, Land Raiders, etc, in your list example there, so the best you could bring is a Knight and hope they don't get too many 6s to glance on multiple facings, with those Flyrants--and it's not like the Knight can do anything to the Flyrants by way of reply, or anything.
I'd start with a Heldrake with a HAC and a quadgun--though they're only AP4, they should put some wounds on the Flyrants by weight of numbers. Beware Vector Striking with the Heldrake, because though it's a good tool for finishing off a badly injured Flyrant (D3 hits that wound on 3+ with no saves? Yay!), you can also expect them to turn around and dump their 12 twin-linked S6 shots into your AV10 rear if they survive the Vector Strike.
I really like Be'lakor, though you need to take care to hide him--even with Shrouded for a 2+ jink save, you can expect him to be the center of Flyrant focus-firing, unless you safely cover him in Invisibility--and with 3 Flyrants, your opponent will be able to get a goodly number of dispels dice to throw at Invisibility. So hide Be'lakor behind some sort of LoS blocking terrain T1, if at all possible. He might make for some decent AA, especially if you've burned your opponents' dispel dice on Invisibility: he could Vector Strike a Flyrant for a shot at some wounds, then follow it up with Psychic Shriek.
That said, you're unlikely to have too many power dice in a a Khorne army list, and you need to ally Be'lakor in already just in order to be able to field him, since Daemonkin aren't a Daemons or CSM codex list, they're their own thing. Be'lakor and 3 nurglings isn't half bad of an allied detachment, though...
Plaguebearer Poisoned is great for killing Nid MCs (on the ground at least), but I wouldn't take them on account of biovores and Mawlocs: if you want them to cross No Man's Land before T4, you have to DS them, and Slow and Purposeful means no running to spread out right after that tightly clustered DS in.
With Daemonkin and your collection of minis, you can't out-Skyfire a triple-flyrant list, so going the 2 Thirster, Be'lakor list would be a seriously uphill battle all the way. That said, you do have one advantage they don't: if Be'lakor manages to cast Invisibility on himself (on top of his 2+ cover) and Shrouding when Bloodthirsters are within 6", they can jink for 2+ cover too. That means less Skyfire that isn't snapping shots on anybody that the enemy Flyrants manages to force to jink, but makes your FMCs a lot harder to put down than his--as long as they stay tightly clustered. Well, I guess to have a knot of them triple-Vector Striking single Flyrants, you might be able to deal with that foe...
A concern is number of Warp Charge, with only 3+D6. You might want to consider taking 11 Pink Horrors as Be'lakor's allied Troops, if only to fund Be'lakor's power-casting.
Going full MSU with as many small units as possible sounds tempting, but... has the downside that if a Flyrant can wipe out one small unit per turn, and your opponent has 3 Flyrants... I don't care how many units you bring, that's 15 units dead by T5 from his HQ choices alone. It's not a very viable counter, I think, unless most of your army is stuck in close combat the whole game so he can't shoot at it.
If I were playing with my own collection, I would probably bring about 35 Flesh Hounds in 7 squads, just to Scout up and assault the enemy and tie up his army T1 or T2, so he'll only get one round of Flyrants shooting at those 7 units before they're stuck in, but... that's on me, not on what you have that you're able to bring, and that's a lot of minis to buy.
What sphinx of plascrete and adamantium bashed open their skulls and ate up their brains and imagination? Imperator! Imperator!