1) I would ignore the time for loading the splunk interface (which should not depend too much on the data) (or you mesure it with no data and add it later if it makes sense to you)
2) For each panel, use the splunk rest api to call directly the search/reporting command -> each request should finished when it's loaded
3) Build a scenario where you fire these 3 requests. (with a valid auth token by the way)
4) mesure time between launch of the 3 rest api search commands and when the last one finished
5) multiply by 3 users
Ps : it may not be realistic to loop without a pause before relaunching a user/view but it also depend on how much time it takes to have a panel data.