Will I gain weight?

First, terminology: BMR (basal metabolic rate) is the number of calories you'd burn in a coma, flat out in bed, not even digesting food or anything like that. TDEE (total daily energy expenditure) is the number of calories you burn doing everything you do, whatever that happens to be. (TDEE includes BMR, daily life stuff like job and home chores, and any intentional exercise you choose to do, plus some other small stuff.)

If you eat at your BMR, but you do things other than lie inertly in bed - like have a job, do some home chores, etc. - you'll lose weight. How fast you'd lose weight varies with how much moving around you do - just the daily life stuff, plus any exercise you decide to continue doing. Unless you still have a lot of fat to lose, eating at your BMR is probably a bad idea.

If you eat right at your TDEE, you'll maintain a steady weight. The trick is figuring out what your TDEE is, since it's different every day (by a little or a lot, depending on your lifestyle). Averages can work, though.

You can gain, lose, or maintain weight while doing any amount of intentional exercise, from zero exercise up to (in theory) every waking hour doing exercise. To accomplish that, you just have to figure out how much to eat, so that you're eating less than you burn in total (to lose), more than you burn (to gain), or close to exactly the amount you burn (to maintain).

BMR (from calculators, MFP, fitness tracker, whatever) is an estimate. In a metabolic lab, you can pay $$$ to get a pretty accurate measurement of your RMR (resting metabolic rate), which is close to BMR but not exact. Most people will be close to the BMR estimates, though.

TDEE is more complicated to estimate. You can get close enough for practical purposes either the MFP way (log and eat back exercise calories explicitly) or the TDEE way (using a TDEE calculator outside MFP or by synching a fitness tracker to MFP).

Either way, you'd want to keep logging and watching the scale trend (not every daily scale twitch!) to experimentally dial in your right calorie level to accomplish your goals.

If you don't enjoy exercising 3 hours a day, it would probably be a good idea to cut back, at least. It's important to have a routine we can sustain relatively happily long term, a routine that allows enough time and energy for family, job, social life, home chores, non-exercise hobbies, and anything else that you personally need in order to have a happy, balanced life.

If you want the body of a concentration camp member then by all means eat below BMR.

I think you just mistook BMR for TDEE though