Insert character after every n characters (backwards)

// Write a function that takes two strings (a and b) as arguments
// Beginning at the end of 'a', insert 'b' after every 3rd character of 'a'
// Return the resulting string
function
myFunction
(
a, b
)
{

return
}
Test Cases:
myFunction('1234567','.')
Expected
'1.234.567'
myFunction('abcde','$')
Expected
'ab$cde'
myFunction('zxyzxyzxyzxyzxyz','w')
Expected
'zwxyzwxyzwxyzwxyzwxyz'