i did the same thing for a school prodject. If you have a nice computer radio, the mixing should be done in there because it will save you a bunch of money. however, if you do not have access to the four channel tx, you will have to go the vtail route. cheap ones will not work here. the only one that i know is accurate enough to work and has adjustable atvs on all output channels with channel offset is the veetail omni. check it out at www.veetail.com
the only real issue there is the fact that it costs $53 for one of them and it would require three of them to do it. here's the setup



you havea 4 channel receiver. you also have vtail mixers that i will label #1,#2, #3,

plug the slave of #1 into channel 1
plug the slave of #2 into channel 2
Plug the master of #3 into channel 3
plug the slave of #3 into channel 4
plug the master of #1 into one of the outputs of #3
plug the master of #2 into the remaining output of #3.


that will give you throtle control, and the aileron/elevator will do the correct thing by slowing down one motor and speeding up the other. however, for this prodject you will need counter-rotating props to save the weight of a thruster. i know where you can get these. that setup will also allow you to control aileron, and even rudder. the rudder will work by increasing the speed of one set of motors and slowing down the other to keep the thrust wehre it is but one pair os going faster so the torque will spin the model. you will have to have a setup somewhta like this. my contraption actually flies, so i am not leading you with theories. if you are interreested i can tell you where to get some of the critical components and whta to avoid