Thank you kipper just wasn't sure , I have not been able to progress the boat for the last 9 or ten weeks due to being out of action with my spine, I hope to get on now after the op last week but I don't think it will see the water till next year. So I intend to get the RC stuff sorted out over the winter

Not necessarily, it would depend on the amount of current your 3 motors would draw & the voltage + ah rating of the battery.

For instance, my tug has 2 motors - 2 esc - 2 sla batteries, they make 2 separate systems, port & stb, so if I lose one side, I can still tug with the other. They would both run just on 1 battery if that's all I put in the boat, but using 2 batteries saves me having to use a lot of lead for ballast.