I mean I've got a friend doing all three extensions: 3U English/Maths/Extension but she only does 11 units and was not able to pick up 4U maths and she's doing fine but it honestly depends if you can handle the workload or not, it's personal experience. In saying that, none of the 4U math kids chose 3U History because they were encouraged to choose one rather than both extensions, I don't know much about 4U math other than its really difficult and a lot more challenging (well nah) but if you genuinely love maths and perform well, you can undertake the 4U math course. As for History Extension, you're most likely going to do this offline, so staying back after school, you've got the major work which is about 3000+ words but keep in mind this goes on for like 9 months and you'll be learning about 'what is history' which you allows you to define history and it'll teach you critical thinking/evaluative language as well as your case study, honestly if you love history and don't mind staying back, by all means go ahead and choose history ext but basically you could give 4U math+ 3U history a shot, then if you absolutely cannot handle the workload, drop one of them, if you can, keep it. Good luck!!