Sorry for your loss.
I don’t think it would be too soon for Moo (some bunnies re-bond very quickly after the loss of their mate), but you are correct in that trying for a trio risks disrupting the current pair.
I don’t have any personal experience with trio’s, but the method depends on the bunnies and how much help you have. Some people like to have the sessions with all three together, other’s like to pair them up first (especially if they don’t have a helper to supervise). Both Mikey and Bianca have bonded trios/quads. I’m wondering if you could do some pre-bonding with the bonded females and Moo, and see how they react when they realize Moo is in the area?
If you are OK with having two separate bonded pairs, your safest bet is to find an unbonded bunny for Moo. Do you have a shelter nearby that will let you date him?
Whichever you decide, the standard method is to let the new bunny settle in for a week or two, and do a few weeks to a month of pre-bonding (cage swaps) before starting bonding sessions.
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.