Eventually you'll level up all the crafters for the cross-class skills. Leveling up a crafter is always an expense that later you recover. The more classes you have, the easier you level the others since you'll be able to make the mats for the crafts needed for the leves.

You need to watch the market board and find what items you can make easily, sell quickly and at good price, and hopefully don't have a lot of offer at that moment. If some item sells a lot, it's not that bad there are a lot of offerers, since it quickly gets sold. If an item does not sell very quickly, having a lot of offerers may mean the price will go down and down before you can finally sell yours. Best look for another one to make. Sometimes an item does not sell quickly but you are the only one making it and it sells for good price, that's other option, until more people sees the chance and starts to make it too.

Of the three classes you mention, I would think GSM looks better to make gil, at least before very high level. All classes wear accesories, and you can start earning gil before you are high level.