I did all three this year. I've finished my Ancient and Modern exams and just have Extension left on Friday.
It's a lot of work to keep up the amount of work needed for all three, but I think if you have a passion for history it's not too bad.
I didn't get majorly stressed through the year or anything so it is an achievable workload.
I finished 1st in Ext 3rd in Modern and 4th in Ancient in my school, so I think it is possible to do well in all three if you like history enough.
It is a demanding subject though, 80% of our class had dropped it by the end of the year, but it is rewarding handing in the project at the end of the year and knowing it's all research you've done yourself.
Good luck if you do end up taking it