This is a very common topology and I can tell you that if you have there configure Spanning-tree (no matter which version) you will be loop free.
STP knows how to block a port on the triangle to keep your topology free of loops. Of course this port will be unblocked in case there is an outage that affect other port, so there will be no interruption in your traffic.
For more information about STP I would recommend:
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_configuration_example09186a008009467c.shtml
http://www.cisco.com/image/gif/paws/10556/spanning_tree1.swf <- graphical example like your case
Calin
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