Out of the box, Word's Mailmerge facility can only really use a single
datasource - in essence a single table of data. But if for example you are
trying to use MailMerge to produce one docu ent per person, and you have
data that specifies for each "Person ID" the doctor, the dentist, and the
school, you may be able to define a query or view that brings all the
relevant data together. That also depends on how the data is stored  - e.g.
if it is in separate Access tables in the same Access database, as long as
there is a suitable link, defining such a query may be fairly
straightforward. If it is in different tables in different Access databases,
or in completely different types of data source, it is likely to be much
more difficult.

You /might/ also be able to create a mailmerge that brings in the data from
/one/ of your sources, then use DATABASE fields to get data from the other
sources. Again, the feasibility and difficulty depends on the data sources
and the kind of data you need to retrieve.

Or if you can get the data using Access, you may be better off defining an
Access report to produce the output you need.

If you can give us some idea of the structure and location of the data you
are trying to retrieve it might be easier to be a bit more specific. It's
always handy to know what version of Word you are using, too.

Probably simplest solution would be to consolidate all data into a
spreadsheet and merge with that.