How to Split 18 GB xml file to 3 smaller files using Python?

I need to split a large xml file (18GB) into 3 smaller files while maintaining the original xml structure. All elements in the file fall under a single root element.

I've been successful using the readlines() method, however, I'm wondering if there is a better way to do this using Python.

My code thus far:

with open(r'largefilepath.xml', 'r') as myfile, open(r'release_sample2.xml', 'w') as outfile:
sampleslice = myfile.readlines(100000)
for line in sampleslice:
    outfile.write(line)