i plan to do a long detox diet of only quinoa, avocado and strawberry.
Why? Unless you have a specific reason to be so limited, this plan sounds so self restrictive that it's likely to fail not from lack of nutrition but from boredom due to lack of variety.
what vitamins/minerals will i be missing if i do this very long term like 6 months.
B-12. Possibly iodine if you're already low.
Minerals: Quinoa has a generous amount of minerals raw according to USDA data. I notice the cooked version, considerably less (throwing cooking water out perhaps?). If you sprout your Quinoa and just lightly steam it, you retain the minerals and most of the vitamin content (as well as added vitamins that appear when sprouting)
Vitamins: I can't see any missing just using these 3 foods except for B-12. As mentioned above, sprouting will increase the vitamin content as well, and introduce some that aren't in Quinoa (like vitamin K) but are in avocados.
If you must boil Quinoa with too much water (which means dumping some of it), I'd suggest Nixtamalization with wood-ash or lime, which will greatly add all the needed minerals in large amounts and some trace ones as well. This actually applies to any grain and other seed type foods like legumes or seeds.
what food do you suggest i add?
If you absolutely feel you must use just these 3 foods, you can, and you'll be fine. However, make sure you get enough calories by eating enough of them.