It depends on your installations and exactly what differences there are between each one, but roughly...
Let's assume you've got 3 installations named EU2_A, EU2_B, EU2_C. One of these will be identified in your registry and Installshield as the "current" installation so when you download the patch and apply it, only this one one will be updated. Let's say it happens to be EU2_A.
The easy way to update the other two installs would be to simply copy all files in the EU2_A folder over to the appropriate folders of EU2_B and EU2_C. You would then have 3 identical installations since this will, of course, overwrite any changes you've made. You would then have to make whatever changes you want to the other installations again - either by editing the files once more or by making backup copies of them in advance.
A more time-consuming method would be to copy the updated files one by one to your other installs, figuring out what you want to do if this results in one of your modded files being overwritten.
A third method would be to remove all three installations first, then do one install and patch, then do a second install and patch, and then a third. As long as you patch after each install, the patch will be applied to the most recent install (but you'll end up with 3 identical patches again that you'll have to mod to achieve whatever changes you want).
The best method depends on what changes you've been making.