The fields from the three phase lines
mostly cancel each other out,
so their combined field is much less
than the field from a single line
of the same voltage and current.
The only strong fields they produce
are in between the lines,
where the energy
is actually being guided along.
Almost all of the fields produced by a transformer
remain inside the transformer.
Especially if it is a three phase unit
(connected to three lines)
again, because of the cancellation effect
of the three phases.
An EMF meter,
that will give you a rough measurement
at various distances,
is not a very expensive piece of equipment
to buy or rent.
I'll bet that your own appliances and wiring
are producing stronger fields,
inside your house,
than these lines and transformers are.
Pull your main disconnect
and all that remains
would be from the outside sources.
Presently, there is very little evidence
that line frequency fields
have specific biological effects.
It is more of a worry
that something might, someday, be discovered.