Too much head space?

I have been reading around on here and once again there seems to be a large dichotomy. I make 3-4 gallon batches and use a 7 gallon plastic bucket as my primary. I usually rack to a secondary but i no longer feel that is necessary for certain styles. Am i able to leave that 3-4 gallons in the 7 gallon bucket for the entire fermenting process (3 weeks) without imparting any off flavors? i dont open until i am ready to siphon.

You will be just fine. 3 weeks is the recommended time to leave ales in primary. It would be a bad idea to rack that to a 7 gallon secondary due to the amount of oxygen head-space in the large bucket. Generally you want to secondary in a vessel that is only slightly larger than your batch of beer to minimize air head-space.