Pond filling time depends on many factors such as if you hit a spring or not, how much runoff area you capture, current weather patterns, etc.
My excavator didn't think our pond would fill until April/May after digging last October. It filled by December, only 3 months after digging. 16 acres of runoff into a little more than half-acre pond will fill it quick when you actually have enough rain for runoff.
As far as swimming, you will most definitely want more than 3 feet in any swimming areas. When the pond lowers by a foot or more in dry summer weather, you may be looking at a dry bottom just when you want to use it the most. Plan on a gently sloped side that extends down to 10 feet or so, and then you should almost be guaranteed swimmable water depth. Or if a gentle slop is not an option, build a dock and ladder to reach greater depths and avoid walking the muddy bottom.
