Parsing an XML file with elements for keys and values
This isn't the answer you want. But, _ is most commonly the discard variable.
You use it when you don't want to use the variable, but you need to specify a variable.
Take:

for _ in range(10):
    print('hi')


We don't want the output from the range, we just want to print 'hi' ten times.
And so you should not use this for the variable you use .clear() on.

I'd use zip rather than the try-except.
Sure there's Look Before You Leap (LBYL) and Easier to Ask for Forgiveness than Permission (EAFP).
But you don't need to use either here.
Also, by using zip you can change your while loop to a for loop too.

And so you should be able to get something like:

import xml.etree.ElementTree

elements = {}
xml_iterator = xml.etree.ElementTree.iterparse('file.xml')

for (_, elem1), (_, elem2), (_, elem3) in zip(xml_iterator, xml_iterator, xml_iterator):
    elements[elem1.text] = elem2
    elem1.clear()
    elem2.clear()
    elem3.clear()