Can I connect PL-501v1, PL-501v2 and PL-501v3 together?

Yes, you can connect them together.
Please notice, PL-501v2 and PL-501v3 are both using INT6300, but PL-501v1 is INT6000, if you would like to connect PL-501v1 with v2 and v3 together, you need to use the utility version v3041B to upgrade firmware to v3052 or above version, so that they can communicate with each other.
Please link to http://www.planet.com.tw/en/product/product_ov.php?id=489 download PL-501's firmware and utility for upgrading.

Note: Please make sure your PL-501 type, and then download the correct firmware version.
FW-PL501v2_3052_US.zip This firmware is only for US version.
FW-PL501v2_3052_EU_UK.zip This firmware is only for EU & UK version.