Is it OK for one side of lip to remain harder than the other 3 weeks post lip filler? (Photos)

If you had hyaluronic acid (HA) fillers like restylane and juvederm, I expect the lips to soften over time. The HA injectables are designed after a natural chemical in the body, so allergic reactions or permanent nodules almost never occur. Other fillers like, artefil or radiesse, are associated with about a 10% rate of permanent nodules or hardness.  
So the answer to the question depends on the type of injectable used.
Have a discussion with your injector to identify which filler was used. If HA was used, then the hardness should disappear. If another filler was used, you might have to try to discuss ways to diminish the hardness. Sometimes the nodules diminish, but they may never go away completely.
Good Luck!

It is't Ursula for the lips to feel hard immediately after lip injections. This is from the swelling. But at 3 weeks post treatment, the hardness should be gone or mostly gone. Not getting worse. I would suggest that you book an appointment with your surgeon to have this looked at. 

This is something that really must be examined in person by your surgeon. In general, HA fillers become softer with time. Not harder. Best of luck.