What is a good 3 man set up with trust support for me and my partner he is currently corsair and I leveled thief but I’ve seen monk thrown around. I will be leveling and dboxing a white mage as well.
WHM + COR forms a solid core (no pun intended), so you really just need to fill it out with any DD and you're good. THF is relatively easy to use, so if you have gear for that, it works. MNK is obviously tankier, and has a lot higher dps potential starting out. Much of MNK's useful gear can be bought right off the AH after 100 JPs, so that is also a plus. The weapon choice is also not too difficult to make, since Ambuscade weapons exist. Personally, I'd go for MNK since it is a bit easier to use at this stage, but THF is perfectly fine as well. I don't know what other jobs you have available, but honestly, just about any DD can fit in a 3-man setup just fine, so long as you're getting appropriate support. You could go PLD here, but I think for what you've described, the need for a harder hitting DPS is better than a heavier tank, IMO.
As far as trust combinations go (nobody has mentioned this), Cornelia is a mandatory. Since you have a real COR and WHM, there is no need to summon either of these trusts, so personally, I would go for a RDM like Koru-Moru or King of Hearts, for Haste2, Dia3, and Phalanx. This will magic cap at least the party leader for everything. Your third trust could be optional based on what you're doing, but I personally think Sylvie is the best DD support trust, period. She heals and -nas super fast, always prioritizes you, re-Hastes you if yours falls (and may even haste other party members), and uses Indi-Fury + EnGeo-Frailty on you, when available. She also survives hits better than most other mage trusts.