Yeah, I would do a flush. Dump everything out, then refill it with plain PH'd water and dump that out. At that point, if your plants are having any kind of problems you should just let them feed on another batch of PH'd water for 2 or 3 days, then dump again and start feeding. Otherwise, proceed as described below.

Also how often do i add the 3 teaspoons into the res.?

Never. Don't ever add nutrients to your reservoir unless you know what you are doing as far as EC and PPM. Unless you have a multimeter monitoring the PPM and PH in the res, you would have no way of knowing how much nutrients or PH up/down to add. It is much much safer to mix up your solution then add it. As the plant uses the nutrients the PH and PPM are always changing. Every time you add solution, you are adding to a solution that is already in there, and each time what is left is higher and higher in salt content and other bad stuff. That's why you need to flush about every 3rd feeding or at least once every three weeks in that size res.

OR

If you don't want to worry about what the PPM or salt content is in your res, you can always play it safe by feeding twice normally, then every third feeding is just PH'd water. This should help keep the salt content and what not low enough that you don't have flush nearly as often, maybe roughly kinda sorta every 8th feeding instead. Personally I prefer more flushing, and therefore more feeding nutrients instead of a watered down version of an old nutrient solution.

As far as how often do you feed? Depends, but I have a similar system right now but on a larger scale, and I just let the nutes run as low as I can without drying out roots too much, then feed. If you have an air stone or wand, you can run the nutes lower since bubbles breaking will splash moisture on the roots. A few days ago that was once every 3 days. Now it's more like once every 2 days. As they get older they eat more and faster.

Oh, and I would just not start right out at full strength with a new nutrient. I use Grow Big, and it seems like 3 teaspoons per gallon might be a little much for even full grown plants, and younguns want even less.

Merry Christmas. Darn I gotta go wrap 2 presents still.