How can I thin old thickened latex paint?

I have 3 year old can of paint that has become very thick. I am using it to do some very detailed edging and I need to thin it. Paint thinner did not work, I tried it on a small amount.How can I thin old thickened latex paint?
put a little water in itHow can I thin old thickened latex paint?
have it shaken where you bought it. if it was stored correctly it should be fine. if its really thick and water based latex you can use water or flotrol but some people still prefer to use paint thinner even though its for oil based paints