Completing the ATPL theory exams in 3 months of actual study time is quite possible but it's certainly hard work. However don't confuse actual study time with total time taken.
For example:
I spent about 11-12 weeks studying the books at home. However the total time from start to finish was 5 months.
Why? Firstly you have to take in to account the actual time to sit the exams, that's one week per module. Then factor in the two brush-up courses at Bristol. Add them both together and that's 6 weeks. This assumes you get the dates that you want for the brush-ups. Bristol is a busy place and many people book months in advance. I couldn't get one of the dates I wanted which added 2 more weeks to the process. And just like that my 3 months became 5 months.
It could be done in 3 months total time but you would need a lot of ability, determination, luck and forward planning.
P.S
I would recommend Bristol Ground School without hesitation - the best in the business by far.