Not pretty, but it seems to work:
numberFormat(floor({GenericSim/Writeable/WriteableFloat1} / 60),"#0")+":"+numberFormat(60*(({GenericSim/Writeable/WriteableFloat1} / 60)-floor({GenericSim/Writeable/WriteableFloat1} / 60)),“00”)+":" + numberFormat(60*({GenericSim/Writeable/WriteableFloat1} - floor({GenericSim/Writeable/WriteableFloat1})),“00”)
Let me know if you want the parts broken down and explained.