Since I never was into multiplayer (too many people I wouldn't want to meet IRL), the only thing interesting me is single player. And since ME3 was rather on the short single player side, I hope they don't continue on that road.
I've never done multiplayer for the same reasons, but I didn't think ME3's single player was short.
15 main missions
19 meaningful base game assignments by my count (N7: Cerberus Labs; N7: Cerberus Attack; N7: Cerberus Abductions; N7: Cerberus Fighter Base; N7: Furl Reactors; N7: Communications Hub; Aria: Blue Suns; Aria: Blood Pack; Aria: Eclipse; Citadel: Hanar Diplomat; Citadel: Volus Ambassador; Arrae: Ex-Cerberus Scientists; Attican Traverse: Krogan Team; Grissom Academy: Emergency Evacuation; Kallini: Ardat-Yakshi Monastery; Rannoch: Admiral Koris; Rannoch: Geth Fighter Squadrons; Tuchunka: Turian Platoon; Tuchunka: Bomb)
A Bunch of other side stuff. I thought S&R was pretty fun, a lot of the ambient conversation parts are entertaining as well.
And the DLC assignments are for the most part very good as well. It's lame at least two of them were DLCs (From Ashes and Leviathan), but they add a lot of content.
I don't remember an exact time for how long it normally takes me to complete a ME3 campaign, but I never felt like it was significantly shorter than ME or ME2. If I recall it seemed to me that each took a little over 30 hours for the base game. I could just be remembering it wrong. It was for the most part worth the price of admission for me as far as length. Now if by "short" you were referring to quality rather than run time, that's a matter of preference and I mistook your original meaning.