Mark,
When sizing step-down transformers, normally bigger is better. For three AB800s with modeling lights, I would recommend a minimum 750 VA transformer. The transformer may saturate, which will cause its output voltage to sag, but the Bees will tolerate that very well. A 1 kVA, or 1.25 kVa transformer would be even better, with less buzzing and line sag from either one. Try to get one with a circuit breaker instead of a fused unit. You'll find it easier and more accommodating to the high initial current surge the Bees will present to the line when starting their recycle period.
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