Can you request a change of location after two years of your three at a duty station?

I was told at each duty station you can request a change of locations after two years of your three. Is this true? I tried a compassionate reassignment to take care of my mom whos very sick and it got denied due to not enough evidence for them. So my next opportunity is to request a station near her. I have someone currently watching her but Its a temporary fix. I get the army way of seeing things so I'm not saying my issue is more important than anyone else. Just weighing my options here and can use a bit of help from someone more knowledgeable. Thanks in advance everyone.