Ectomorph, endomorph, or mesomorph? please help!?

First of all yes. You can be a mix, but it is not simply ecto-meso, endo-meso ect but degrees of each. From what I can tell you have very high ecto tendencies. Like you said your small joints point to that. Mesomorphs gain muscle in a completely different way and tend to maintain it very easily. Not that you can't be muscular but you would have a hard time building it. Especially describing you diet. When I eat that much (but better choices) and workout I tend to gain ten pounds of muscle. So based off height, weight, joint size, calorie consumption, and muscular description, you sound like a full ecto. Definetly not endo. One final indicator is abs and chest. Ecto- flat chested, very defined abs but not projected abs. Meso- pecs are rounded and extrend from chest, abs project, and generally have a v shape around hips. anyway good luck hope you figure it out.