I assume that you are going to get a bank letter that contains all the required information except it does not have six months average balance for your account(s). I suppose you have one of two possible issues - a) your account is less than 6 months old or b) your account is over six months old, but you only had the funds in place for 3 months.
In scenario a), the bank letter will state the date that your account was opened, so it's self evident that you can't have a longer history of balances. Since you must submit the documents within 60 days,I guess you could wait as long as possible to get ~5 months of history.e In scenario b), you should just show six months of balance (whatever it may be). You may be asked for further information in the second case, but giving only three months will raise red flags.