We are a global company looking to implement SAP RISE, with entities in numerous markets (15+).  SAP are suggesting a regional approach. Three separate RISE Instances to match our regions.  However, if we are trying to ensure consistency in processes globally, is there not a way to have a single Global instance of SAP RISE?  So we can benefit from the user scale in the global business, or does local data regulations / bank integrations get in the way of achieving that?

You need to look closely at whether a single instance is possible.  Regulations like data sovereignty may make it difficult to achieve this.  Also, think about how the system will be used.  If the system physically resides say in London and users are in Australia, Japan and California, the user experience may not be that great.  Even at the speed of light things do take time over these distances.  Also, think about how other systems will be integrating with this single global instance.  Machine to machine interfaces over thousands of miles all over the globe introduce are something to think about.  Similarly, if users such as CEO, CFO etc. look at dashboards or similar things on their tablet that is cellular connected, what will that be like?  How about maintenance time?  If users are on the system 24 hours per day (follow the sun), which region will be impacted when a 4 hour maintenance is needed by SAP?  There is no "quiet" time in a system that "never sleeps"/users are always on.  In summary, yes, technically it can be done.  Once done, what will the non-technical result be like?  A little NJ humor....To put it another words, yes, we can put a toilet in the kitchen.  Technically it can be done.  How many people want to live in that house?
   

You can absolutely have a global single instance. Local laws and localization requirements can add some complexity, but the long term cost optimization of not having multiple instances and dealing with integrations and consolidation between multiple instances more than makes up for it. SAP will naturally push multiple instances in a RISE model to increase revenues.