The arena will mark the third major project that Comcast Spectacor is now undertaking on the site. It is renovating the Wells Fargo Center for $250 million and seeking tenants for a 10-story office tower, costing $80 million, also near Xfinity Live. The price tag for the three projects, including the new arena, is $380 million. The company may add yet more development on the site, say those with knowledge of the company’s thinking.