I do exactly this, in a very similar situation!
I also play in a three piece band. Our drummer runs backing tracks, and I augment the tracks with soft synths from an iPad. (Incidentally, if it’s pads your after, check out Synthscaper, very deep.)
I use a variety of Widi devices to make it all happen. My pedal board has a uHost, powered by the usb port on a Cioks DC7, connected to the usb in on an MC6 (this also powers the MC6, saving a power port). Another uHost is connected to the iPad. A Widi Bud is connected to my laptop, for editing MC6 presets, and Future Impact presets. This comprises a Widi ‘group’ and allows me bidirectional control between the iPad, the MC6, Future Impact, Source Audio C4, Pigtronix Echolution3, and a Beebo. In addition to soft synths, I use GoButton for sound effects/ tracks, TouchOSC to control the C4 and E3 in real time, and have recently started using Bandhelper, which can send/receive midi as well.
I’ve recently added a Widi Thru6 BT (a 2in, 6out, merge/splitter) and a keyboard as a secondary group. This lets me control everything on the board, and play the soft synths on the iPad. I can also ‘play’ the Future Impact, as the oscillators respond to midi notes. The two groups merge at the Widi Thru6. Everything is wireless except power.
I use the MC6 to trigger pads, and have been using the waveform generators for volume swells, tremolo, reverb throws, whathaveyou. The MC6 really is a tremendous device.
In the past, I’ve run the audio thru a small two input mixer, straight to my bass rig, for ease of setup, but lately have been running into the pa. The drummer has just started using GoButton for tracks, so the next step will be to sync his tracks with my rig.
Sorry for the long post, hope this helps.