Ignoramus29948 wrote:

Would it be legal and possible to use 3/4" conduit to carry two 4 ga
conductors (hots) and one 6 ga conductor (neutral). That would be for
possibly upgrading a garage subpanel with existing 3/4" conduit. Most
bends would incorporate electrical boxes.

i

Nope, (except if you use imc conduit), but you would be much smarter to use
1" anyway - it would be an easier pull.
Look up the conduit fill tables in the NEC, you can even find those online.
Just remember tho, just because its legal doesn't necessarily mean its
smart. ie: sure you can legally put x conductors into a y sized conduit but
if the conduit is very long it gets to be absurdly difficult to get things
pulled in without damage. Using larger conduit makes for an easier job,
reduces the possibility of insulation damage, allows for future growth and
costs very little.
Eric