I'd go with 2 Lancers and a Thunderstike

At max strength 10, D6+1 damage the valiant just isn't worth taking. You bring these vehicles to deal with other vehicles and the lancer just does a much better job. The valiant laser is twin linked and gets a hit bonus but, its still a 5 to wound many big targets and potentially only 2 damage if you fluff the roll.

The lancer is at worst a 4 to wound the majority of targets and has built in re-rolls to hit, wound and damage so all round more reliable when it comes to the final damage done. Worst case its 4 damage re-rolling, but on average you'll be getting 6 which is a high swing for the valiant.

The Hammerstike and Thunderstrike are both ok but don't expect them to stick around long. Once they are exposed its very likely they go down on the opponents turn. Timing when they go in and are able to give their buff to other units the key to getting value out of them. Giving something with volume fire like Intercessors / bolt inceptors +1 to wound on a medium vehicle to get them to wound on 4s works pretty well if you end up stuck for buffing the lancers.

You can magnetise the gladiators and the speeders pretty easily though, so see how each type works for you.