Out of the three...I would choose the PST. The Medallion tote is very nice but for a first piece (personally) I'd get something with the signature Chanel chain...I wanted to get the Medallion first too but after seeing it along with the GST IRL, I decided to go for the GST!
The small timeless tote (PST with zipper) that everybody is talking about is nice, kind of shaped like the GST except it's much smaller, is VERY small. I saw it yesterday and was disappointed because it's quite a small bag for the price...it costs around the same as the Medallion I believe.
Whatever you choose, good luck!