yes it is normal, I know many people on SSI/SSDI

a CDR (continuing disability review) happens periodically to everyone on SSI, usually every couple years to every 5 years depending upon what the reported disability is. if you look back on your forms and paperwork you got when you were first awarded SSI you will find a spot about reviews and how often you will have to participate in them. you can also find this information on your pamphlet social security sends out to those on SSI, most get their SSI information pamphlet yearly as the laws and such are changed.

when they send you a form it means they just want to know if you are still disabled to the point that meets the criteria (which changes often) for receiving SSI in your location.

when they request someone to see treatment providers its in the recipients (person who gets the SSI) best interest because most people on permanent SSI usually end up having more health issues as time goes on.

example you said you have been on SSI now for three years. thats three years worth of new health issues you and your treatment providers can include on the forms that were not the application three years ago.

they require medical and mental reviews also because SSI is not a program limited to just medical problems or just mental problems.they also include it in the reviews because some mental and physical health problems can be cured or maintained to a point where a person who has a mental or physical health problem can go back to work.

you can choose not to get the evaluations requested but sometimes that means they either consider you treated to the point where you can go back to work and deny future checks to you, or they base it on your file which may no longer meet the new guidelines for receiving SSI.

Also here in america we have recently switched over to new mental disorders and some of the older mental disorder labels no longer apply as diagnostic labels of mental disorders. that means SSI has since 2013 had to change their listing of what mental disorders and such they cover. example someone with DDNOS (just using this as an example because I know someone that this happened to) could be denied SSI because here in america there is no longer a mental disorder called DDNOS. by going to the treatment providers required a persons mental disorders can be updated to the present diagnostics of OSDD or ODD (which is in part what DDNOS used to be)

my point its in your best interest to get the new evaluations done so that your mental disorders and health issues on SSI files are up to date, that way you dont accidentally end up in a situation where your past file no longer meets todays standards for receiving SSI.