The ideal angle would be 45°, since the weight is then taken evenly on each strand with the least stress. In those circumstances, a 400 lb load would have a strain of 200 on each side. If the central angle becomes 120°, it doubles, and at 150°, it's 800.
Double-wrap the cables, and maybe slip some wood between them and the load to stop them slipping.
Three-point hookups are not common, and are usually for loads designed for them, but boats are lifted this way. 4-point slings, on the other hand, are quite common and used for box-like loads, attached to each corner—where the sling may catch or damage the load, use spreader bars to keep the rope away, as well as for stability:

Beware of loads with a high C of G, as they may tip over.
Tag lines (short lines underneath a load) are for loaders to grab on the setdown. They should have a safety latch and be heavy enough to stop waving about, but long enough to catch if the load is swinging too fast.
Argos and Quads can be moved with strops underneath. Just wrap a strop around the Quad engine, ensuring it goes inside the pedals (or they will get bent). Then choke it and attach it to the hook.
Personnel briefing
All concerned should be aware of:
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The hook-up

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The setdown

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Hand signals

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Proper use of radios (i.e. don’t hog the airwaves)

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Direction to move in case of engine failure*

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Not standing under the load

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Number of trips between
refuelling stops

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Retrieving slings and nets

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Use of protective equipment

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Accident procedure

*This depends on which way the rotors spin. With North American rotation (anticlockwise as viewed from above), the helicopter has a natural tendency to drift to the left when the engine stops, so ground crews should always work to the right of the helicopter and be prepared to go that way if they think anything untoward is happening. Astars go to the right.

Hooking Up
Ground crew are nice to have, but you often have to do it yourself:

Note: Always lay it out in front!
If you have one, the marshaller should be at least 25m from the load with his back to the wind so you can see him from your high position. If he needs to change places, he should cease marshalling first, so he doesn't move backwards into unseen obstructions. Using standard marshalling signals, you will be positioned over the load, where the loaders apply the static discharge probe to the hook and place the eye of the net or sling inside it.
As mentioned above, loaders should always work to one side of the helicopter, and should also keep clear of the exit and approach paths, just in case you have to drop a load.
The loaders then give an affirmative signal to the marshaller, who gives you the "move upwards" signal until the slack has been taken up. You will increase hover height slowly, until the strain is taken, with the loaders guiding the strops as necessary, taking care to be free to move away quickly should the need arise. At all times in the event of engine failure, the ground staff must move in the opposite direction that the helicopter would go, e.g. JetRanger to the left, staff to the right, or the opposite for a Squirrel (it depends which way the blades rotate). They should not turn their back on the load, or get directly underneath it, or the flight path. Neither should they wrap lines directly around their wrists or bodies. When finished, they should clear the area as soon as possible.