I'm an Artificer and I recently dropped Archaeology in favor of Slicing. So far it hasn't been a problem for me to buy my Archaeology materials off the GTN. You also have the option of having the other skill you need on a second character and mailing the mats over. So it's definitely viable to craft with only one gathering/mission skill.
If you go with two non-crafting skills there are a few things to consider. Gathering skills provide more materials than pure mission skills because you can collect stuff as you go around questing. Bioanalysis and Scavenging in particular are great because you're always killing things. On the flip side, while you have more to sell so does everyone else. So the supply is higher and thus the prices you can sell for are lower. However, one of my guildies uses Scavenging and tells me that he sells his mats for very good prices, probably because Scavenging mats are used by 3 crafting skills (see the answer linked in the comments). Slicing is useful since it's pure credits, particularly with the new Section X area for level 50s that has a lot of the high-end lockboxes, and mats from its missions also feed 3 crafting skills.
Mission skills, and missions for gathering skills, are the only way to get low-level purple materials which sell for the most. However, you only get these when the high-yield missions crit or when you get the purple Mission Discoveries and use them. Usually only the latter are worth the investment, but it depends on how much they sell for since you won't find enough of them on your own.
In the end, you should see what various materials and crafted items are selling for on your server before making your decision. Some mats definitely sell for less than they cost to acquire so there's no point in running missions for them, and there's various degrees of profitability for others. When it comes to crafting: Cybertech is the strongest skill for selling, with Artifice in second, simply due to how many endgame items they can craft. Biochem can be very useful if you're using items for yourself.