Unfortunately, with timechart, if you specify a field to split by, you can not specify more than one item to graph. This is because, when you split by a field, the distinct values of that field become the column/field names. In essense you want to produce a graph with 3 axis (time, host, and AvailableMBytes/Context/PercentProcessorTime), but we can only produce 2D tables and charts.
You could try doing this (basically flattening the 3rd axis together with the host field):
index="wmi" host="hostprefix*" (wmi_type=CPUTime OR wmi_type=NET OR wmi_type=Memory)
| kv
| fields wmi_type,host,PercentProcessorTime,Contexts,AvailableMBytes
| eval pivot = "PercentProcessorTime;Contexts;AvailableMBytes"
| makemv pivot delim=";"
| mvexpand pivot
| eval x=if(pivot="PercentProcessorTime",PercentProcessorTime,if(pivot="Contexts",Contexts,AvailableMBytes))
| eval host = pivot . "-" . host
| timechart max(x) by host
... View more