Do you think sars-2 will mutate into sars-3 in a year or it will disappear and reappear after 20 years
Your question runs into problems with the proper terminology:
SARS-CoV-1 (SARS) and SARS-CoV-2 (COVID), are separate human-infectious strains* of the SARS-CoV virus, with a 79% genome overlap (ie significantly more different from each other than we are from Chimps at 96% overlap), SARS-CoV-3 would be used for a third crossover from the parent lineage, not a variant of SARS-CoV-2.
(MERS is MERS-CoV, a separate species of Coronavirus, within a separate sub-genus (Merbecovirus vs Sarbecovirus), but the same genus (Betacoronavirus)).
We've already seen the emergence of multiple variants of SARS-CoV-2, but accumulating a 21% divergence from the original genetics equivalent to the difference between SARS-Cov-1 and 2 will take a considerable time (coronavirus mutation rate seems to be estimated at about 25-35 base pairs/year). The spread of vaccination will drive evolution within the different Covid strains we've already identified, because strains able to do better in vaccinated individuals will have an evolutionary advantage, but that only takes fairly minor genetic changes - coronaviruses contain about 30,000 base pairs (DNA characters), but the mutations that have gotten attention such as E484K (aka Eeek!) actually denote a single codon change (from one set of 3 DNA characters coding for a single amino-acid to a different set of 3 coding for another amino-acid), the smallest degree of change possible.
I expect SARS-CoV-2 to become endemic, because there doesn't seem to be the political will to wipe it out. The point at which it might just vanish was missed way back at the start of 2020. There clearly is the potential for new coronavirus crossovers, because we've seen three in twenty years, but there's a good chance they'd come from a different coronavirus than SARS-CoV or MERS-CoV.
* There are also bat-resident strains.
