Check engine light keeps flashing off and on and when I change gears it sputters out and doesnt gain power, what is causing this?

you have a misfire condition. never drive a vehicle with the CEL flashing/blinking or severe damage can/will occur. a little low on oil,it only holds 4.5 qts including changing the filter.I know you said you just bought it,but keep an eye on the oil level.have the codes read and post codes here,someone will help you,and most parts stores will read the codes free of charge.

As has been suggested, a flashing check engine light typically occurs in a misfire situation. For clarification, did you add 3 quarts of oil, or 3/4 of a quart? If you added 3 quarts, you were very low in oil which could cause a myriad of problems. I would also suggest you get the vehicle scanned for codes as they will at least point to a symptom of the problem, if not the problem itself.