I would say go ahead and sign up for all 3 and diversified your options.

On face value, Roo probably pays less than Grab. However, Roo allows you to freely cancel incoming orders, while grab will punish you for doing it. On the weekends, the time spent at DTF waiting to pickup your grab order, you could had cherry pick and do 2 roo orders instead...

For us part timers, the game differs. Full timers benefit sticking mainly to one platform for the incentives. For part timers, imo switching platforms is best. I prefer doing Roo at peak hours for the flexibility, then switch on grab after peak. I don't have experience with Panda, but based on the signature sheet you see at merchants, panda seems to be on par and sometimes, even more orders than Grab...