Workflows for Tab like functionality

so now if user submits Panel A1 it should trigger a workflow

you can not submit single panel or tab, it's possible to submit only whole form.

How to make fields required on the form according to panel  for example if user clicks Tab A - Fields in the Panel A should become mandatory.

you have build validation rule with formula like

TabControl == "TabA" && IsNullOrEmpty(NamedControlFromTabA)

I thought the workflow should be started from the form - not that the workflow is already running.

Just like Cassy suggested I would create a hidden field containing the current tab.

Then I would create three workflows A, B and C corresponding to the tabs. Then set the workflows to start automatically and conditional based on the hidden field.

I cannot use this formula

you can not check panel control for emptiness. panel is just a visual container for other controls, it doesn't have value on its own.

check my formula above - you have to check value of control you want to make mandatory

and can I not make all the field s from the panel mandatory??

sure you can.

select all the controls you want to make mandatory and assign a rule to them.

set rule formula like

TabControl == "TabA" && IsNullOrEmpty({Self})‍

so instead of reference to specific control use generic {Self} reference, it will be evaluated at runtime to a value of control currently being validated