Assuming there are three switches interconnected(Switch1--Switch2---switch3---switch1) to each other and STP has convergence has taken place and there is no change in the topology for a while. Now my question is that to converge STP BPDU's were flowing generated by all switches in the network, But after the convergence are BPDU also flowing (after convergence and before any change in topology) ? And if they are flowing all three switches are generating them or some particular Switch?
The root bridge sends BPDU's to know if the network has changed and to inform other bridges in case of change.
The frequency of those BPDU's is determined by a setting called hello time that can be set on the switch. Usually is two seconds, a value low enough to guarantee a good convergence time and also high enough to not overflow the network.
Yes. In short , during convergence and election, all switches will originate BPDUs. Once converged, and root has been elected, only the root will originate it. Remaining switches will continue forwarding it though.