what are you trying to accomplish?

if you want to have a single folder, SUBSITE, and three files, 1.html, 2.html, 3.html, then you can of course, but you will have to type the full URLs:

• username github io/SUBSITE/1.html
• username github io/SUBSITE/2.html
• username github io/SUBSITE/3.html

if, instead, your goal is to have these three working URLs:

• username github io/SUBSITE/1
• username github io/SUBSITE/2
• username github io/SUBSITE/3

then you can simply put your index.html files with yet another subfolder, each. e.g.:

/SUBSITE/1/index.html
/SUBSITE/2/index.html
/SUBSITE/3/index.html