Can I still pay for my vehicles license sticker after being past due 3 weeks?

Yes i believe you still can

Your vehicle registration may be renewed online 90 days before the expiration date or up to six months after the expiration date, if you have not received a citation for expired registration. Your expiration month will not change. Go here to the official site: TXDMV.GOV - Register Your Vehicle https://www.txdmv.gov/motorists/register-your-vehicle

If the vehicle is more than 1 week past the month due, HEB will NOT allow customers to purchase vehicle registration stickers, you will need to go to the nearest tax office to pay for your vehicle's registration.

Yes