List poolmembers that are down,unavailable and Unknown

Since you are using the value of "Availability", you can do the following: add the operator "-or" like:

$poolmembers | Where-Object {$_.Availability -eq 'AVAILABILITY_STATUS_RED' -or $_.Availability -eq 'AVAILABILITY_STATUS_BLUE'}

or just filter any different like: $poolmembers | Where-Object {$_.Availability -ne 'AVAILABILITY_STATUS_GREEN'}

Is this what you want?