In The Sign of Three, when Sherlock is having a conference with the 5 women (the Mayfly Man case), he finds that the employers of all five women are different. So how does Tessa know about the wedding and why did the Mayfly man date the other women?
SHERLOCK: Job.
GAIL: Gardener.
CHARLOTTE: Cook.
TESSA: Private nurse.
ROBYN: I do security work.
VICKY : Maid.
SHERLOCK: Obvious. You all work for the same person!
SHERLOCK: No, not the same employer. Damn.
Source for Transcript: Ariane DeVere
TLDR:
There are two possible reasons how Tessa knew about the wedding: she saw the invitation whilst working for Sholto, or was told by Sholto in her capacity as his private nurse.
The Mayfly Man dated the women as they all worked for Sholto at some time.
Long Answer:
All five women have dated the same man (The Mayfly Man), named as such as he lived like a Mayfly:
The man had given a different name on each occasion...He'd taken the names of people from obituaries. He'd found single recently-deceased men who had lived alone - and he'd taken the women back to their homes. So on each occasion, he'd taken a dead man's identity for one night only.
So to answer your questions as to why he dated them - the most logical explanation is that they all worked for Sholto at some time.
Now, we know from the conference that Sherlock initially dismissed this link as they had different employers. Instead, he focussed on the fact they all had a secret. However, there is an issue with this. Whilst they all work for different employers now, their employment history is never mentioned.
From the script:
At the reception, everyone is back in the room. Sherlock tries to act nonchalantly as he walks over to a nearby table and picks up one of the name cards on it while pulling a pen on a chain from his waistcoat.
SHERLOCK: Ooh! A recluse, small household staff.FLASHBACK TO THE COUNCIL CHAMBER.
SHERLOCK: Job.
GAIL: Gardener.
CHARLOTTE: Cook.
TESSA: Private nurse.
VICKY: Maid.SHERLOCK (writing on the name card in the reception room): High turnover for additional security.
FLASHBACK TO THE COUNCIL CHAMBER.
ROBYN: I do security work.SHERLOCK (walking over to Sholto and casually dropping the name card down in front of him before walking away): Probably all signed confidentiality agreements.
He realised at this point that they had all worked for Sholto at some point (not necessarily at the same time). They either lied about it to Sherlock as they had signed confidentiality agreements or they were simply never asked by Sherlock as he was more interested in their current employer.
So the photographer who tried to kill Sholto effectively dated all the women due to their connection to Sholto.
As for Tessa, there are a few possibilities as to how she knows about the wedding. Note her job - private nurse. She has a strong reason to know Sholto's whereabouts at all times, in case he needs medical attention. It's possible she was told he would be at the wedding and passed this information on during her seduction by the Mayfly Man. Another possibility is that as Sholto's private nurse, with close access to him, she simply saw the wedding invitation lying around and likely made the link between the name on it (John Watson) and the blog she reads online.
