Your collection makes for some fun and flexible lists.
You could run of your tacticals as a 10 man sternguard squad in your drop pod, cbat squad when they disembark so that you can stand on 2 facings of the knight. Loaded out with meltas and combi-meltas, that would drop one knightalmost every time.
Try running multiple chainfist in your terminator squad (3-4), they will usually die, but should drop a knight, if it is damaged some before they go in. I assume they ride in the raider, make sure you take that multi-melta on it.
Run your tac squads as 10 man squads broken down into a 5 man squad deployed in cover with a heavy weapon, and 5 more in a rhino carrying a meltagun and sergeant with combi-melta (can you tell I llay Salamanders?).
That is probably what I would run IF I played with your collection against a 3 knight list. HOWEVER, I would recommend you just don't. Playing against 3 knights is not very fun even with a list that stomps it (accept for that dark joy I feel inside when they are removed from the table), so, for me unless I am lined up against them in a tourney, or we had a fun narrative game put together, I would just pass on playing the guy.
Unless of course he is willing to run only 1 knight, as thats not a big deal and everybody has a good time.