The F5 specifications spreadsheet I have from April 2013 rates the 2000s at: 5M max concurrent connections, 75k L4 cps, 25k L7 cps, 5Gbps L4/L7 throughput.
(That's overall bit less than the 3900, which is 6M concurrent, 175k L4 cps, 50k L7 cps, though only 4Gbps L4/L7 throughput.)
These are raw TCP numbers, and likely under ideal conditions: everything using PVA where possible, no SNAT, minimal processing. There are various reasons you might see 100% CPU well before the above numbers - all do to with what you're doing with those connections such as HTTP profiles, iRules, compression (there is no hardware compression on 3900), etc.
Some more information on your configuration around these connections might help to size the right replacement appliance.