Despite being a junior you have a lot to offer and most chapters will realize that! I think you should definately mention that you're going to be there for at least 3 more years. With that fact mentioned, it makes you just as eligible as any sophmore rushing. Put an emphasis on how much you want to get involved with positions within the sorority and activities outside.
With respect to Monday meetings, they are probably new member meetings, which are absolutely mandatory. Usually if you don't attend new member meetings you don't get initiated. Ask each house while you're going through what happens on Monday nights, but don't necessarily say you have a class that you can't change.