@brownr 

solve({eq1, eq2, eq3}, {i1, i3, vo})

then we get vo by knowing i1 and i2 

vo := (Z*ro*vs+Rs*Z*A*vi+Rf*Rs*A*vi+Rf*Z*A*vi)/(Z*Rs+Rf*Rs+Rf*Z+ro*Rs+ro*Z)

after that we divide by vs

vo/s

however from the picture below we see vs appeared in the answer....

@Markiyan Hirnyk 

Sorry Zi = Z