The last three cards in the Buffy season 3 #s 87, 88, 89, are cards Featuring Angel and advertising the Angel Season 1 release.
Having nothing to do with Buffy but numbered and issued in the Buffy 3 set.
Would these cards be considered as Promo Cards

I wouldn't think so in MHO. Season 3 is billed as a 90 card set with #90 being the checklist. I would consider #87 - #89 to be necessary to complete the base set. Not a bad idea for advertising a coming product though.

For me these cards are part of the season 3 set and not promo cards.

That was my thought but on reflection.
The three cards are numbered in the set thus being part of the set
But they have nothing to do with Buffy except being crossover cards and pictures of Angel advertising the upcoming Angel 1 set of cards
Was just a thought that they could be classed as Promos

No because they are #'s 87 88 + 89 of the Buffy S3 set
wouldn't hurt if you wanted to include them to your Angel collection though.