I first would suggest you try to see all three of them in person and try them on (with and without coats) and see how you like them. I've pined over bags that I thought I'd love until I see them irl and while they're still fabulous, they just don't look good on me
Between M/L and Jumbo, if you love both and have to choose one, then perhaps the Jumbo would be, not necessarily better, but strategically a good choice since they're adding another flap on the inside starting Cruise 10/11 and will no longer be producing the Jumbo with just the single flap (I'm no expert so if I'm wrong here, please Chanel experts kindly correct me). Grab it while you still can I suppose is the motto here
I personally love the M/L more than the Jumbo because of its versatility from day to night, casual or business. Also, I feel that the Jumbo overwhelms me sometimes (even though I'm not a petite girl) but I've seen plenty of girls smaller than me carry it off fabulously. But the M/L style is not going anywhere and the design isn't getting changed, so perhaps you can wait on that for now?
Now onto the Lanvin, it's much more affordable compared to the Chanels, but if you don't love it, it wouldn't matter how much more or less it costs. I love the Happy mainly due to its less than structured body. The bag is amazingly spacious and I think I listed on here somewhere how much I could fit in it, and it's quite a bit! The taupe color is quite amazing in person and suits my wardrobe. I particularly love the ribbons and detachable coin charm and the vintage gold/brass looking chains, the chevron quilting, the mineral/stone like hardware, it just spells romance and vintage to me. With that being said, it can come off as 'fussy' (the removable coin charm usually doesn't get used because I don't like to hear that cling sound when I carry it to work) and not as 'clean lined' as I've heard from a friend. Is it classic? I'd say so, even more so if you get one of the classic Happys.
Investment wise, I think the choice is obvious--Chanel, partly due to their constant price increase, and they're more well-known. I do think Lanvin will receive more attention from the masses after H&M's end of November collection hits stores. One is one of the most recognizable(if not the most recognizable) Paris fashion house, and another is the oldest. At the end of the day, it comes down to which one YOU love the best!
Hope this helps