I'm trying to sell the coins on coinbase. I already have an account with coinbase. I'm just not sure how to send to coinbase and what the bitcoin addresses that start with 3 have to do with the transfer.
P2SH address format is a bitcoin addresses that starts with number '3' and is supported by coinbase, that is why you have given a deposit address starting from 3 in your coinbase account. Also, you really don't have to worry about sending your bitcoin from your legacy address (address starting from '1') to P2SH address (coinbase) since it is widely accepted and can be used to deposit funds from both legacy and bech32 (address starting from 'bc1') address format.
What you can do is to follow the steps provided by other members before me in importing the paper wallet's private key to a verified electrum wallet[1] so that you can transfer your bitcoin to your coinbase account and start selling your bitcoin. More importantly, you need to be aware that the paper wallet is no longer secure and can be considered as compromised after importing your private to another wallet since paper wallet is intended only for single use.
[1]
https://bitcointalk.org/index.php?topic=5240594.0