I am a 65 yr old female. I can hold my planks for nearly 3 min with a quick 10 sec rest in between. Why is my stomach still pouching out.
Ability to hold a plank is determined mainly by the strength and endurance of your abdominal muscles. Having a protruding stomach is determined mainly by your level of body fat. The two are not related, so you can have strong abdominal muscles with a high body fat level, and you can have a low body fat level with weak abdominal muscles.
Additionally, muscles do not specifically burn fat from the area around them (this is known as the "spot reduction" myth), and so planks are not useful for burning belly fat.
It sounds like you are putting too much emphasis on Planks. Planks are a good exercise, but it is just one exercise. A well-rounded workout should include dozens of exercises. Also, Planks are a stability exercise for your lumbar spine, but they are not an effective way to burn carbohydrates (or fat). To burn carbohydrates, you should do exercises with more movement.
On a separate topic, if you are doing a plank for three minutes, then I can almost guarantee that your form is horrible by the last minute. If I were your coach, I would cut the time back and focus on excellent form. Can you take a picture of what you form looks like 150 seconds into your plank? I'd be curious.
Assuming you have good form, if you can hold planks for 3 minutes it has probably become less efficient as an exercise for your abdominal muscles. It's a good exercise for beginners but for a strong core you may want to introduce exercises with movement. They challenge your core muscles better as you need to use the muscles to stabilise your body.
You could try some variations that will introduce movement to the static plank, like:
- (side) plank dips
- (side) plank knee tucks
- plank hip dips, or
- grab small weights for some one-arm rows while holding the plank.