12. The number in $\triangle$ is 3.1
12.【Solution】To make the average as small as possible, we should use smaller numbers more often and larger numbers less often. For these five $\bigcirc$, the numbers in the $\bigcirc$ at the ends only participate in one operation, so they should be filled with 6.5 and 4.6; the number in the middle $\bigcirc$ participates in three operations, so it should be 1.2, and the remaining two circles should be filled with 2.9 and 3.7. In this case, there are two ways to fill them, but no matter which one, the calculation shows that the number in $\triangle$ should be 3.1. Thus, the number in $\triangle$ is 3.1.
12. As shown in the right figure, fill the numbers $1.2, 3.7, 6.5, 2.9, 4.6$ into five ○s respectively, then fill each $\square$ with the average of the three $\bigcirc$ numbers it is connected to, and fill the $\triangle$ with the average of the three $\square$ numbers. Find a way to fill the numbers so that the number in $\triangle$ is as small as possible. What number should be filled in $\triangle$?