Using an L298 to drive 2 unidirectional and 1 birectional motor simultaniously
Yes, you can drive one bidirectional and two unidirectional brushed DC motors from a single L298. Just connect each unidirectional motor from Vs to one output of the same bridge (eg. OUT3 and OUT4) and you can control it from a single input. 

The only limitation is that since there is only one current sense line per bridge, you won't be able to independently monitor each motor's current draw when both are running.