I've been eating healthy for about 3 1/2 weeks now. watching nutrition and cut out processed. is frequent urination common with diet change?

It is not uncommon!: The thing that seems to cause diuresis (increased urination) for most people with diet change is the significant reduction in sodium that comes with eating healthier/avoiding more processed foods. Often, when people lose more weight the first week or so on a "diet", it is chalked up to "water weight." Your body soon equilibrates. Remember, healthy drinking of more water also leads to more urine:)