Like JT says, plus possible c of g issues. 'All 3 switches on' is fine in practice when the intention and application is to load correctly, but this is why ALL fuel loading should be monitored by qualified personnel to ensure that the centre tank is switched off at the right level and that the wings load the while. The only problems I ever had were in TLV once with a 734 with aux tank where the fueller was loading the aft aux before the rest which was a strict no-no, and with BA's liking for putting a calculated amount into the centre BEFORE the wings are full which on a couple of occasions meant I had to over-fuel to get the wings full.
I guess in strict accordance with the limitation it would be correct to load wings first.