If 3 days is what you have to work with there is no reason why you can't make it beneficial to gaining mass. Ask yourself, are you more likely to gain mass working 3 days a week, or working 0 days a week? I bet you're seeing the benefits already.
I would say combine Back & Chest; Arms & Shoulders; Legs seperate.
"Heaven and earth shall pass away, but my words shall not pass away." - Matthew 24:35
"But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith, meekness, temperance: against such there is no law." - Galations 5:22-23