PC1 is connected to Switch1, and PC2 and PC3 are connected to Switch2. How can PC1 and PC2 not communicate with each other, but both can communicate with PC3?

If they are L3 switches, this requirement can be achieved through ACL.

If they are L2 switches,it can be achieved through vlan.For this requirement,It can be divided into three VLANs.

For example:
The port for PC1 connection is set to vlan100,300 untag; Set pvid to vlan 100
The port for PC2 connection is set to vlan200,300 untag.Set pvid to vlan 200
The port for PC3 connection is set to vlan100,200,300 untag. set pvid to vlan 300.
Set the interconnection port of the switch to trunk mode with vlan100,300 tag.