I have a family that has a 3 year old female, got from me at 1 year.
In the past year she has shown signs of skin allergy and he says anal glands have to be expressed every three weeks????

eating pro plan chicken, and adding sweet potato for fiber.

any suggestions that would cause this or help fix it? Vet is talking about removing the anal sacs

I would try a food change - maybe to ProPlan Sensitive Skin and Stomach. Two of my dogs have had anal gland problems in the past, and both cleared up with food changes. It may take a few different foods to find the right one, but the ProPlan SS&S is a good place to start.

It could be environmental allergies like grasses (allergies are not primarily caused from food)

I also had a girl like this that once spayed did not have allergic condition

you can do a blood test for allergies that is somewhat accurate, but expensive but at least they would have some direction to take

I had a girl with similar issues years ago (she also had pollen allergies). After a year or so of routinely having her gland expressed, at the suggestion of her breeder, we started adding 1tsp of metamucil to each of her meals (be sure to wet down the food if doing this). We also added some canned green beans with her dinner - both for fiber and for weight control. Eventually we switched to Benefiber instead of metamucil as it dissolves easier, but from when we started on this regimen at about 3 or 4 years old, to when we lost her at 15+ she never once needed to have her glands expressed.

after talking more with the owners I have learned.

there is no allergy issues, the vet suggested that the anal gland issue are an allergy issue.

coat and skin are in excellent condition, she weighs 65 lbs, eats 3 cups of Pro Plam All Stage a day.

I did find out they add water to her food, and adding sweet potato to her diet.
I had them stop the water, possible that the stool is not firm enough to release the glands naturally.

they are going in every 3-4 weeks because they fill up and leak. I am wondering if the non stop expressing has them inflamed and thus blocking them? I told him to learn how to do this technique so he did not have to go to the vet every time.

My older boy (7 years old) was a frequent 'sqeezer' @ the vet - every 3 weeks! The vet told me that these frequent gland expressions will eventually get more difficult and with that they may not be able to express all of the gland material which can become infected and may abcess. I opted to have the glands removed along with spaying him (no relevance to the anal gland issue). Recovery went well, 1 Tbsp. of Metamucil in morning/evening kibble during recovery.

All is well that ENDS well

You're right! I think 'NEUTERING' is the more appropriate method here! DUH!!! That's what I get for not previewing and editing BEFORE I post my message. Funny Friday!

Thanks for keeping me honest
(I hope I have it right this time)