Type numbers from 1 to 9, letters A to J, and numbers 00 to 09 to briefly see what symbol is hiding. To match a pair, type the corresponding number followed by the corresponding letter (eg if you see the Euro symbol next to '1', 'A', and 01, type those in any order, like 1a01 or 01a1) The checklist will help guide you along the way.
Maybe it's just my computer or eyesight, but I can't make out the symbols as they appear - there's not enough contrast between the green background and the symbol to be able to figure out what it is.
Clever idea adding a third column so that you can't just make it by guessing all combinations! The symbols aren't that visible, perhaps increasing font size would be of help