There is no duplicate detection capability in CRM 3.0. You can achieve it with an add-on product, like c360 (www.c360.com) , or use Scribe Insight (www.scribesoft.com). The import utility in 3.0 is very limited--it will import duplicates if there are duplicate records.
There are two compelling reasons in your question to upgrade to 4.0:
1. Very strong, configurable duplicate detection.
2. Significantly improved import utility.