[j0hnik]

str1 = '&par1=1&par2=RR&par3=234,223&par2=A';
str2 = '&par1=2&par5=pp&par2=2';
str3 = '&par6=2&par5=pp&par7=2';

var filterURL=(...arg)=>
arg.join('')
.match(/&[^=]+=[^&]+/g)
.filter((e, i, s)=>s.findIndex(el=>el.match(/&[^=]+/)[0] == e.match(/&[^=]+/)[0]) == i)
.join('');

console.log(filterURL(str1,str2,str3));