I routinely have to request cell records are preserved in anticipation of getting a court order and most carriers do not keep the actual texts more than a few days. Some rare carriers keep text content do but most, in my experience, don't. You have to make sure you get them preserved the minute you think they may be important.I'm not sure where you heard it was 3 days - Florida subpoena'd Casey Anthony's records far later than that. Three days may be what's available to the customer but a court could get them to go back much further than that.
http://www.theverge.com/2012/12/3/3722996/police-ask-congress-preserve-sms-logs-two-years
Police call on Congress to require that carriers maintain SMS logs for two years Dec 3, 2012
"Carriers generally store rudimentary data for past messages (the numbers involved, a timestamp of when the conversation occurred, etc.) for several months, but the actual content of those chats can be permanently lost within days. A Justice Department memo publicized last year shows that Verizon stores detailed logs for between three and five days while AT&T, Sprint, and T-Mobile apparently keep no such records at all."