No stool for 3 weeks must i be worried?

Usually yes: Each person with constipation needs to be sure there is no other disease causing it. If constipation is just due to slow movement of the food and poop through the gut, then the things to try are vegetables, water, prune juice, prunes, fiber+water, Miralax (polyethylene glycol), Senokot or other senna, milk of magnesia, other laxatives... A doctor can evaluate and prescribe helpful medication too.