I am wondering if a machine that currently has Microstation SS4 and Inroads SS2 installed can run Geopak SS3.
Yes. Please see this wiki for further discussion: https://communities.bentley.com/products/road___site_design/w/road_and_site_design__wiki/19018/can-geopak-ss2ss3-be-installed-on-the-same-machine-as-geopak-ss4.
For more information about the Road and Site design tools, visit the Road and Site design WIKI at: http://communities.bentley.com/products/road___site_design/w/road_and_site_design__wiki