A small apartment complex. 12 buildings with 6-12 apartments.
Three of the buildings have periodic brown outs (lights dim momentarily) for no obvious reason. Tow of the buildings share a transfromer. the third has it's own.
It does not seem to happen when something in the buildings starts (but I would not totally rule it out).
What are possible causes?

There are many. Startup of a large load, possibly on another "load" on the primary of the transformers.

A call to the POCO, may be in order for starting point.

Next, a basic "bad" connection, secondary side line or neutral.

As you say 3 buildings, I would start there.

Could be a bad feeder from one building to the next. Maybe the 3 buildings are the last on line.