I wasn't thinking about side-slip when talking about air-flow, but of course you would need ports on both sides for that purpose. I happened to look at some photos of 757s and found one facing the other way, and indeed there are matching ports on the starboard side:
ET-AMT - Ethiopian Airlines Boeing 757-200 at Delhi - Indira Gandhi Int | Photo ID 351638 | Airplane-Pictures.net This photo also has the advantage that it was taken close to perpendicular to the body, thus allowing more accurate estimation of the location of the ports relative to the windows.
I also came across this report which states there are three ports on the left side:
ASN Aircraft accident Boeing 757-23A N52AW Lima, Peru
It seems there must be 6 ports, though the photos appear to show only 4. I wonder where the other two are. It seems likely that there are two in one of the darker areas, possibly the larger, more forward one, which appears to be elongated vertically.