I’m going to build a bike for my wife

She’ll share my cx wheels with 11-27 ultegra cassette

I’m going to put flat bars on with 3×10 shifters (maybe shimano deore)

Do I need a mtb deore/slx etc rear mech (medium)

Or

Will a 105 road rear mech be ok

anyone?

nope, not as far as I know. It’ll need to be dynasys, offroad 10 speed mech.

will mtb levers & mech work with a road cassette

will mtb levers & mech work with a road cassette

yes

can get ultegra flat bar trigger shifters too, and i’d expect a 105/ultegra rear mech ought to work on that cassette (so long as you get a model with enough capacity for the 3x up front).

the mtb shifters are cheaper than the flat bar road shifters though

I need a

3×10 chainset
3×10 front mech
10spd rear mech

to work on a ultegra cassette

3×10 chainset, 9 or 10 speed chainrings will work with a 10 speed chain.
never been convinced about the difference between 9 and 10 speed front mechs either.

yes, 10 speed rear mech needed.