After draging the ports of three sources(sql server,oracle,informix) to a single source qualifier, can u map these three ports directly to target?

if u drag three hetrogenous sources and populated to target without any join means you are entertaining Carteisn product. If you don't use join means not only diffrent sources but homegeous sources are show same error.
If you are not interested to use joins at source qualifier level u can add some joins sepratly.
In Source qualifier we can join the tables from samedatabase only.