That makes sense, what I meant by having up to 3 after you is I've seen them buzzing around and they never interdict me, then I leave and jump to the next system and get interdicted by a different assassin, we start dueling and the first one shows up but doesn't attack me until I'm finished with the one I've been fighting. They get in the way of our fight but I'm careful not to hit it until I'm ready.I've had it sometimes that they would keep radio silence or somehow manage to disappear from my contacts list right after that message.
Though the fishy message is also quite common amongst pirates.
I think you are referring to the assassins always coming in groups of three. Which means, for every initial message you get from your contractors that someone is sniffing for your exhaust fumes, there are three of them, but you only ever see one of them until you kill that one. Then the next takes over.
But as you can stack those missions (I usually took 20 of them at the starting point of my trip, creating some kind of roundtrip, picking up new missions on my way.), you might get more than just one message, up to five, which makes 15 assassins in total.
I've had had it happen like that, up to five assassin groups following me around on my courier trips. Which can be fun if your destination is a little further into the system - I once had seven interdictions in one system, five assassins and two pirates. No chance to recieve any rewards for the extra speedy delivery...
And in my experience, if you have so many sassasins on your six, the best way to survive would be evading them, as I died a few times trying to fend off al lower ranked assasin (smaller ship / less reward for deleting them), only to have a big gun in an Anaconda jump in mid-fight and blast me out of the sky before I completely realized what was going on.
When I was doing the massacre contracts, my notoriety plunged and they would open up on me as soon as they found me, up to 3 attacking at the same time (thus, cell banks are a must have). They would ambush me everywhere, nav beacons, extraction sites (sometimes NOT welcomed by pirates from different factions) and when I left supercruise to enter a station. One of the funniest I saw was at the megaship Dove Enigma, Colonia system, an NPC cop barracks/system control ship. Right after they attacked me, they were swarmed by NPC cops, I was in rough shape from a previous interdiction so I moseyed on over to ship and landed to make repairs. Anyway they are a lot of fun if your setup for it and know what to expect.