Cannot SSH into Cisco switch or pfSense router from Mac Mini (macOS High Sierra 10.13.6)
Try this:

ssh -oKexAlgorithms=+diffie-hellman-group1-sha1 192.168.3.2
ssh -oKexAlgorithms=+diffie-hellman-group-exchange-sha256 192.168.3.1


If I'm reading it right, you've set the cipher for 192.168.3.1 (router) to diffie-hellman-group1-sha1, but it only offers diffie-hellman-group-exchange-sha256 in the DH family.

More info on OpenSSH legacy connections here. Related to that, SHA1 ciphers have been deprecated for good reason - they're considered unsafe or broken, so it would be pertinent to investigate whether your router or switch software can be updated to a modern, more secure version.