C, 209 203 137 bytes
-66 bytes thanks to ceilingcat
char a[9];main(f){gets(a);f=strlen(a)-3;a[f+1]=0;printf((*a==55&a[1]==49&f==1?a[1]=98:*a==49&!f?*a=98:a[f]==98|*a==48&!f)?"%s":"%sb",a);}
C,
girobuz
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