Trade-In iPad Pro 11" Gen 3

Apple generally don’t list current models for online trade-in. As such, you might reasonably infer that trade-in of the now-current model will become available when a successor/replacement model is launched - whenever that might be.

Apple does not pre-announced its product development or release schedule. As such, you yourself might speculate on the basis of previous product releases.

As the option is not available online, perhaps visit your local Apple Store - and discuss your options. Otherwise, sell your iPad privately.