can i trade in my current apple watch as credit towards the apple watch series 3?

Hi

No - Apple does not currently offer either a trade-in program or an upgrade program for Apple Watch.

You may wish to consider selling your Apple Watch privately and putting the proceeds towards your new purchase.

If doing so, be sure to follow Apple's instructions for unpairing your Apple Watch and turning off Activation Lock, such that it is left ready for a new user to set up as their own:

About Activation Lock on your Apple Watch - Apple Support

Best Buy now accepts the watch in the trade in program. Just looked at the Original Watch, 38mm Silver and they are offering a $91 best buy card for it.

Apple currently does not have a program for trading in the Apple Watch.

And that is a separate program. It is not an Apple program, and as you indicate, you get a Best Buy Card for it, nothing from Apple.