I was working on this project some months ago, it shows the parametric capabilities of FreeCAD.
Almost every parameter of the design is customable from a spreadsheet, if you want to test it, just change de "Cantidad ruedas" (number of wheels) value for a number higher than 3 (the higher the number, the longer it takes to generate the model).
It was designed with FreeCAD 0.16 and uses functions from the Part Design, Part, Draft and Spreadsheet workbenches. In this next days I'm gonna rebuild it from scratch using the new Part Design workbench (FC 0.17), making the model less susceptible to break from parameter changes.
The FreeCAD file and renders are apparently too large for the forum attachments, but everything is on my GitHub https://github.com/gsegura96/FreeCAD-Omniwheel
Hope you like it, please let me know any problem or recommendation.
PD: for now everything is in spanish, the next revision will be translated to english.