The TIPS Thin Bed Modeler is a great tool for rapidly comparing hydrocarbon beds for Wet, Oil and Gas conditions at the same time. You never have to worry about the fluid substitution process as it is handled automatically for you as you make changes to the environment.
At all times, the 3 conditions just move along in tandem whether you are looking at AVO, Wedge models or comparing Seismic Attributes.
Sometimes you may want to just compare 3 random beds that have some other relationship besides the hydrocarbon content. If you select Model + Reservoir Controls from the main menu, you can do just that.
Rather than reading all about it (even though its in the software documentation), just watch this video and see how its done.