Try this:
| makeresults | eval data=" yes,a,host1,category1|yes,b,host1,category1|yes,c,host2,category1|yes,a,host2,category3|yes,b,host2,category2" | makemv data delim="|" | mvexpand data | rex field=data "(?<event>[^\,]*)\,(?<myField>[^\,]*)\,(?<myHost>[^\,]*)\,(?<myCategory>[^\,]*)$" | table event myField, myHost, myCategory | stats count(eval(myField=="a")) as A, count(eval(myField=="b")) as B, count(eval(myField=="c")) as C by myHost, myCategory
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