SC2Mapster

I am trying to make players start with 1 CC, 1 Supply Depot and 1 Barrack. That is easy to change in the "Races" tab in the data editor.

The problem is that the structures sometimes spawn among the mineral patches, depending on the map layout. See picture link: http://imgur.com/a/7ZH0y

Any ideas how to make structures ALWAYS spawn on the opposite side from the mineral patches?

This is probably because the barracks and supply depot don't follow the placement rules of the command center where it has to be a certain distance from minerals to be placed. I'm not exactly sure how to fix this without ruining the normal barracks.

You could apply the same placement settings as the command center, but then the player wouldn't be able to place buildings near minerals if you're ok with that.

You could also create a duplicate version of the barracks that is only used by this start mechanism. You should be able to have more than one barracks (just make sure to use the prefix or postfix to label in the editor so you know which one is which). Then have the placement settings of the command center on the special one time use barracks. I believe the placement is based on the footprint.

You could make a trigger spawn the units/structures opposite of the mineral location but if you only want to stick to Data go with

Quote from DrSuperEvil >>

I would just add a Buff behaviour that uses a Create Unit effect that is validated using an Ennumerate Area validator.

You could use a buff as suggested by DrSuperEvil but with a search that uses a nearby mineral field and the caster of the buff as endpoints for the vector of a persistent with an offset to create the units (basically a data version of the trigger solution provided by tdhsst).