Hi Rakesh,
I really like your aspiration and courage to shift your career after you travelled so long in testing.
Like others pointed out so can become a SAP Tester (ONLY if you want to) because it would be easier shift rather than moving into Functional, Why I'm saying Functional rather than Technical because it needs a lot of effort for you from now on to understand the coding , OOPS concept and later learning ABAP language which is called TECHNICAL (unless otherwise you have done automation testing)
Are you working in the same domain (like FMCG, Insurance, utilities etc...) for three years ? If yes, you might have become expert in that domain so got to search for the Functional area which is more likely towards your domain
You can look within your company for SAP testing or try outside, Let's start with this as first step then you like continue else keep aspirations floating and choose a specific Functional area like (CRM , SD, ISU) and move on..
Hope this answer you question.
Feel free to reply back if you have any clarification, I will try to help your pursuit of switching your career.
Thanks & Regards
Rahaman.