Pete25....I understand your $ costs constraints
....but it appears you did not plan & do your $ sums prior to the build
...hence your current $$$$$ quandry
I also understand & agree with the comments from
wartsilaone....so is there a compromise?
1. you could consider running the two outboard motors only...the middle motor could be installed with the prop...but not connected electrically
2. you could consider fitting oversize rudders [two or three] depending on the vessel original design
From here you could consider two ESC + use two channels on your TX...which wil get you into the water & running....sometime down the track when $ funds were available an ACTion mixer could be considered between the two outboard motors only
Not sure on the best search method here, but I have read about 3 engined model vessels [PT style vessels?] where the center engine is only used for high speed work & running without any form of ESC....this facility could be simply switched by servo to micro switch & controlled by another TX single switch function.....
A few things to consider
.....clearly all is not lost
....Derek