No you can't get 3A from parallel secondaries rated at only 2.5A.

If the transformer secondary is rated at 1.25A, as in this case, then 1.25A is what you can take from the transformer without it being overloaded. This is AC, alternating current, unfortunately you cannot equate this directly with the required current at DC, direct current.

There is a factor that comes into play called the TUF, transformer utilisation factor. When rectifying to DC there is also a loss associated with the rectifier topology.
In the case of a bridge rectifier these two factors are equal so the maximum DC current that can be taken is 81.2% of the transformer AC rating.