It is almost as if the decimal were a one. From there it goes to the right. Ie. Tenths first, hundredths, then thousandths, etc. Hope it helps.
The thousandths digit is the third digit after the decimal point and to round 4829.67321 to thousandths we need to see its fourth digit which is 2. Since it is a 2, then we round down. The answer would be 4829.673