This may be a dumb question but I wonder also, do chickens not know their own. The 3 pullets are from the hen's spring batch but she weaned at 6 weeks and went back to where she came from (she didn't belong to me but roamed the neighborhood). She came back 7 weeks ago to hatch this new set and I decided to try to keep her. Would she know the 3 pullets are from her first batch?
No, generally they do not 'know their own'.
Young birds, in this case once the broody weans them, are not really part of the adults pecking order until they reach point of lay.
It's odd that she hatched chicks with you, then went to another keeper, then came back to you to hatch another batch?
Do you have other chickens other than this broody and her chicks?
Maybe you'll find some helpful clues in my notes about....
Integration Basics:
It's all about territory and resources(space/food/water).
Existing birds will almost always attack new ones to defend their resources.
Understanding chicken behaviors is essential to integrating new birds into your flock.
Confine new birds within sight but physically segregated from older/existing birds for several weeks, so they can see and get used to each other but not physically interact.
In adjacent runs, spread scratch grains along the dividing mesh, best if mesh is just big enough for birds to stick their head thru, so they get used to eating together.
The more space, the better. Birds will peck to establish dominance, the pecked bird needs space to get away. As long as there's no copious blood drawn and/or new bird is not trapped/pinned down and beaten unmercilessly, let them work it out. Every time you interfere or remove new birds, they'll have to start the pecking order thing all over again.
Multiple feed/water stations. Dominance issues are most often carried out over sustenance, more stations lessens the frequency of that issue.
Places for the new birds to hide 'out of line of sight'(but not a dead end trap) and/or up and away from any bully birds. Roosts, pallets or boards leaned up against walls or up on concrete blocks, old chairs tables, branches, logs, stumps out in the run can
really help. Lots of diversion and places to 'hide' instead of bare wide open run.