all quests compleded

i have finished 3 acts - how to level my character after this? should i start a game once more? enemies will be scaled to my level?

After you finish act 3, you're returned to act 1 on the next difficulty. If you're stuck in act 3, there's a quest to get a key for opening a locked door that sends you to the next difficulty.

If you're in merciless, then the end game consists of running maps. They're items that can drop randomly anywhere in merciless.