Kender, you might be okay with 5/8" if I understand your design. I'd probably still build it to at least a full inch though. You're not really going to add that much weight or size from an aesthetic perspective, but the added strength with the extra lam will be tremendously beneficial. For 3 adults, let's just average 600 pounds (assuming relatively large males) as a static load. Centripetal force is F=(mv^2)/r. For a 600 pound load (let's use weight as mass for this example even though that's not quite right) and a height of 5 feet for the length of the swing arm (pivot point of support to bottom of swing seat), if I did my math right, that makes a load of approximately 4735 pounds generated by the swing for a velocity that would make a full circle with the swing in 5 seconds. Centripetal force isn't the right calculation for this because it assumes a constant velocity, which a swing doesn't have. Calculation of the torque is probably a better formula, but I don't have that handy at the moment. Either way, it gives you an idea of the types of forces that can easily be generated by a mass in motion, though.
I'm assuming your design would put the 5/8" perpindicular to the downward load (IE in the horizontal plane of the swing) and the 4" measurement is parallel to the motion (IE in the verticle plane of the swing). As such, the 5/8" is really more a factor when considering the lateral motion of the swing (side to side as you sit in the swing). With appropriate bracing along the bottom I don't see why that would be a big issue, because laminated wood is stronger than a single slab and (if I'm correct in my assumptions of your design) you're not really dealing with the major forces of the swing with this particular dimension.
All that said, I'd either bump it up a little to the full inch, or glass it as you suggested. Never hurts to over-engineer a little bit.
Last edited by frankp; 10-15-2009 at 08:48 AM.