any time you add letters to a number you initiate a contraction. Like can�t for can not, won�t for will not; However, when you add to a number you are combining two forms of language that do not correlate. So it should follow literal rule that 3rd is to be written as third, or capitalized Third when expressing a date or time. In casual writting, 3rd is ok as is 4th, but in formal writting accepted by schools, universities or written text to groups it should follow the literal rule without the contraction as a number, or written as third and not 3rd. To informal groups, or singular recipent any form is acceptable.