In a shameless attempt to steal some votes, SecurityProtocol is an Enum with the [Flags] attribute. So you can do this:

[Net.ServicePointManager]::SecurityProtocol = 
  [Net.SecurityProtocolType]::Tls12 -bor `
  [Net.SecurityProtocolType]::Tls11 -bor `
  [Net.SecurityProtocolType]::Tls

Or since this is PowerShell, you can let it parse a string for you:

[Net.ServicePointManager]::SecurityProtocol = "tls12, tls11, tls"

Then you don’t technically need to know the TLS version.

I copied and pasted this from a script I created after reading this answer because I didn’t want to cycle through all the available protocols to find one that worked. Of course, you could do that if you wanted to.

Final note – I have the original (minus SO edits) statement in my PowerShell profile so it’s in every session I start now. It’s not totally foolproof since there are still some sites that just fail but I surely see the message in question much less frequently.