I rarely gig with more than 2 instruments so can get away with just using A/B switches..
but if I was in your situation
I'd definitely suggest this as an ideal
versatile gig-proof build quality stage mini-mixer..

http://www.award-session.com/mixmatch.html

Its specifically designed for inputing electro acoustic & magnetic pickup string instruments.
.. designed & manufactured by a well respected British Amp Engineer;
and costs about £70 brand new.
I'd buy one myself, but just can't find a reason to justify just aquiring one
for the sake of it...

I'm very keen on Award-Session's practical and reliable utility pre-amps,
got 3 or 4 of 'em at good bargain 2nd hand prices, so my enthusiasm is based on genuine experience...