I have 3 RC HV Servos (working voltage 6-8.4V) (link). I purchased a 75 Watt Switching Power Supply (link). The servos draw about 3amps at full load.Is a 75Watt supply sufficient or do I need more just to be safe? Also, is it safe to connect the servos directly to this supply or do I need to add a current limiting resistor between them?
According to a comment in your link, the maximum draw is 3A, so 3 could draw as much as 9A. That's 68W so your 75W supply appears to be adequate, at least to the extent the 3A represents a peak draw. To be 100% sure I'd like to see an official spec from the manufacturer that this represents stall current.
Chances are you'll not be running all three simultaneously at full load so I think you're probably okay. A supply like that (and it is from a reputable manufacturer- Meanwell) should gracefully current limit.
No you do not (should not) put a resistor in series, just connect them to the supply (observing polarity, of course, or you'll probably be ordering more of those nice servos).
By the way, you're apparently using a proper distributor (Mouser) so no worries, but I have seen counterfeit Meanwell supplies on the market so be careful with supplies outside official channels.