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What you might be missing is a 2811 can not handle a T3 at full rate. From personal experience, had a 2811 hit 100% CPU at about 20 Mbps (duplex) across a T3.
Since a 2851 offers about twice the performance of a 2811, it's probably the smallest router that could "safely" handle a T3.
Although Cisco might recommend something faster than your 2951 for a full T3, since it's, I believe, faster than a 3845 which also was recommended for T3s, likely it will work fine.
Since even the 2901 is faster, I believe, than a 2851, any 29xx might be able to handle a single T3.