RE: will 40 size trainer fly on three channels
You can Y the rudder to the ailerons and thus control a 4-channel model with a 3-channel radio.
Its not the best solution... but it works OK on a trainer. Generally, this would give more rudder than is correct for a proper "autocoordinated" turn so a reduced rudder throw would help.
That gives a semblence of the old Ercoupe controls which linked the nosewheel to the ailerons and had no rudder.