How can I trade in my MacBook to purchase a new one?

Go through the Buy process (and trade-in estimate) for the MacBook Pro M3. Without purchasing, it will tell you the expected trade-in amount if any, and the final price of the new M3 device configuration. You will pay full price for the MacBook Pro and once Apple processes your returned trade-in, your credit card will be refunded the trade-in amount.

You can see if your device qualifies here

https://www.apple.com/shop/trade-in

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

Also make sure your T2 chip if present is unlocked:

Mac computers with the Apple T2 Security Chip - Apple Support

Otherwise whoever receives the computer will not be able to use it, and it will have to be sold for scrap.

WE are all other users here and do not work for Apple so we cannot quote prices.

Also, please note: Apple uses third party trade in partner companies in some areas and that would be where you would send it. Take plenty of photos before you pack it because they make the final decision and it may not be what Apple estimated initiallly - unless you are buying/trading in at the store. Apple would then inspect it and make a final offer immediately.

Only Apple can tell you based on their evaluation of the worth of your computer.