can't help but do是固定用法,意思是“只能做”,类似的还有：can't but do.
另外,还有can't help doing,意思是“不由得做.">
can't help to do/can't help do意思是“不能帮助做."
都是正确的,就看所使用的场合了.
但是,can't but to do是不正确的.
希望对你有所帮助.