I'm pretty sure I have this right but want to double check before flying it: if Han is attacking, and I use Etahn's ability to change one hit to a crit, Han's ability world then only reroll the other two dice, correct?

If you want to use Han's ability you must reroll ALL dice that you can legally reroll - including the hit you just flipped to a crit

I know that's what Han's ability is, but my question (I guess I didn't really ask it clearly) is, does the rule on modifying a particular die only one time mean that once it's been flipped to a crit it cannot be rerolled?

I haven't flown Han very much so I'm trying to get a better feel for his strengths

All rerolls are modifications, but not all modifications are rerolls.

A given die can only be REROLLED once, but it can be modified as many times as you like. Etahn's ability is not a reroll, so it doesn't really interact with Han's ability. If you use Han's ability to reroll, you have to reroll all possible dice, since Etahn's ability isn't a reroll, it can still be rerolled and would be included.

Ok thank you for spelling it out

I know that's what Han's ability is, but my question (I guess I didn't really ask it clearly) is, diss the rule on modifying a particular five only once mean that once it's been flipped to a crit it cannot be reerolled?

I haven't flown Han very much so I'm trying to get a better feel for his strengths

a modified dice can be re-rolled - a re-rolled dice can be modified but cannot be re-rolled

In Han's case you MUST re-roll all dice that can be legally re-rolled - a dice modified by Etan's ability can be legally re-rolled - therefore it must be if you want to use Han's ability

In other words - use Etan last