P90X3 and no weight loss

If you lost inches but not weight that should be telling you all you need. You are likely gaining muscle and retaining some water but losing inches.

I would weigh your food though like poster above me suggest if your not doing so already. Also, make sure you are getting adequate protein so that your not risking muscle loss in the long run.

FWIW, I'm starting week 4 of T25. I've actually gained ~4 lbs, & while I'm (stupidly) not measuring inches, I'm definitely losing them. Are you seeing results that way?

Water retention in your muscles to repair. You may also be eating much more than you think you are, do you weigh all solids you consume and measure liquids? If not you may be over consuming by 10-50%.

That said, why are you aiming for such a large deficit?
How much weight are you trying to lose in total?

if you have over 100 lbs to lose you can lose more than 2 lbs/week safely, that being said it is best to set a realistic safe weekly loss goal and:
If you have 75+ lbs to lose 2 lbs/week is ideal,
If you have 40-75 lbs to lose 1.5 lbs/week is ideal,
If you have 25-40 lbs to lose 1 lbs/week is ideal,
If you have 15 -25 lbs to lose 0.5 to 1.0 lbs/week is ideal, and
If you have less than 15 lbs to lose 0.5 lbs/week is ideal.

I'm suspicious that you are actually burning 3677 calories per day. Your calculator may not be accurate If you've lost inches then you are losing fat. You are probably just retaining water because of the new muscle use and repair. Also if you are doing P90x and burning that high of calories I would say this is a bad time for a low carb diet. I don't how low you are going but if you are doing a daily intense workout like that you need sufficient carbs.

at 6'7" 300Lbs, a good 2500-2800 would be BMR alone, with no exercise at all.

bump

I also wear a bodymedia armband and even though I have it set to recognize that my arm is the dominant (even though it's not), it's still high. You can adjust your METs on the website to increase your threshold. After that, just estimate that it's probably still over-calculating for you (I know mine does by about 200-300 calories). It's like MFP and your exercise calories: know that it overestimates and eat back only a portion. The armband is great, but it's not perfect.

I haven't done p90x3, but I HAVE done p90x and my experience was that I didn't notice anything visually until 7 weeks in. I new I was making progress and losing weight, because I weighed in once a week and the workouts were getting easier. But i didn't really SEE the difference till about that time. So keep going, don't discouraged and just keep pushing play. I don't know what Tony's catch-phrases are in this series but my favorite from the original is "Rome wasn't built in a day and neither was your body."

I'm day three of Week 5- and I just started seeing results(weight loss) this week. I didn't even see much of a change with measurements the first month, but my clothes are fitting better, I can tell. It's just going to take some time. I did have to adjust my calories- at one point I was up 4-5 pounds and freaked out. I'm sure most of it was water... but I didn't want to gain that much and 'undo' my previous work. The program says I should eat 2100, I was eating 1800-2000. Now I've switched it down to 1600-1800... and I'm seeing some weight loss. But then, maybe I would have any way at week 5. Who knows?!