would it max my growth if i use lvl 5 population with lvl 3 oil and mineral structure.

no, u should have lvl 5 farm as well as lvl 5 oil and mineral structure. However, ur growth will not (and never) be maxed, it's not only depend on the structure, the number of crystals u have, resource outposts, workers, relics, conquers, and the production boost, etc. are all affect ur growth.

Its better to have max farm over metal and oil structures at a start of a round in my opinion. Your farm gives you a higher worker increase thus giving you more income through workers plus the % of income to workers is increased while the metal and oil structures only increase base amount given.

thanx that would help a lot.