I got this from the Boost site, the description from the $50 unlimited plan. It did not mention 3 way calling at all so I guess it is not supported.

Call or Feature Type: $50 /Month
Days - Unlimited
Nights & Weekends - Unlimited
Mobile-to-Mobile - For calls to & from Boost Mobile®, Sprint® & Nextel® subscribers nationwide - Unlimited
Nationwide Boost Mobile® Walkie-Talkie - Unlimited
Text Messaging - (including Picture, Audio, Video) - (available on select phones) - Unlimited
Wireless Web - Unlimited
Call Waiting - Included
Voicemail, Long Distance & Nationwide Network - Included
International Long Distance See rates
International Text Messaging
FREE to receive; 10¢/msg to send
International Connect (Unlimited Walkie-Talkie & Text) Add unlimited international text and walkie-talkie for an additional $10/month