adding flax seeds to the shake is fine. :) 3 tablespoons sounds like a lot to take in one serving, tho, you may want to take just 1 tblspoon at a time, 3x a day.
You still need to get other fibers from diet. Apples are a wonderful fiber, very easy to pass. And since berries are in season take advantage of those while you can so you get the wonderful benefits of flavanoids, which are great for the immune system. Once they're out of season you have dates, raisins, figs and nuts of course.
If you can handle the sugar, consider drinking a small bottle of prune juice once a week to keep things moving and moist. If you can't, then try a tea mixture and more oils that induce bile production.