I filled an extension out. I filed three weeks later. My tax return still just shows accepted. What can I do?

So you efiled the return?    Double-check with this tool to be sure the return was efiled successfully and accepted:
https://shop.turbotax.intuit.com/efile/efile_status_lookup.jsp

If that showed it was accepted, what do you see at the IRS website below for the status of your Federal refund?
https://www.irs.gov/Refunds
NOTE: When using that tool, use only the Federal refund amount, and not any total refund amount that includes a state refund. The amount to use is on your Federal Form 1040, Line 20a.

If that IRS tool shows that the refund has been sent, but you never received it, report back for more info.

If the IRS tool says it is "still being processed", we can tell you how to speak to a live agent at the IRS to see if they can tell you anything about the delay.

To track a state refund:
https://ttlc.intuit.com/questions/1899433-how-do-i-track-my-state-refund