You can get copper "boluses" if you don't mind cramming pills down their throats
A side benefit is it kills worms too:
http://blog.valleyvet.com/blog/2011/09/using-the-copasure-bolus-for-goats.html
Quote:
They can be castrated at any age, but when older, it's best to do it surgically.
You should be OK with banding at 3 months as long as you can get the band on