I have a character who is a normal human seamstress, she puts on a mask and turns into a highwayman (yep sex change) and she also gets some other abilities from her gear.
Because of what is happening the plot she is now going to get a 3rd form which has a lot of different powers (she is being hired by a devil to reap souls for him). Think a highwayman version of ghost rider less stealth, more combat and an undead steed.
How would you handle this with regards to points, she can only use one suite of powers at a time?
That's a neat concept idea. I'd suggest that, instead of using Instant Change, you maybe consider Alter Ego. I remember BASHMAN talking about how your character's point cost being totaled from the highest stat or power of every form. When any powers overlap between the different "forms", then you only pay for the most expensive one. Alter Ego only mentions two forms but I couldn't see why three forms couldn't work.
You could even tie one of those forms into the Involuntary Change disadvantage (as a balance to the Alter Ego advantage to make one of your forms available only under particular circumstances).
To keep your costs reasonable, try to figure out what makes each form unique and try to overlap a few of the commonly shared powers (at different levels of course).
This would be somewhat difficult to pull off with Street Level heroes and extremely difficult for Mystery Men characters. If the campaign is at World Class levels then this would be a cinch! It would cool if you were to post your new, triple-formed hero (however you decide to build it). I love the concept and would really like to see how you accomplish it.