Hmm, in looking at your post dfk again, you did say sump tank so we likely are talking about a waste tank rather than a product storage tank (I originally thought this was a product tank that could be lined up to different services).
I would still say, unless there is a common cause that would result in all three stream occuring at the same time, that all 3 streams would likely be considered an non-credible event. However, I'd take a look also at how often these events occur at these rates, that should give you an idea of how often the probability is that all 3 would occur at the same time or any 2. If it's once every 5 years, that is quite a bit different than once a month. Without knowing the overall system, it's hard to be more specific. The rates you posted may be maximum, worse case, rates while the 'normal' rates (that could occur more often) may be much less and may not be a concern if both happen together.
You could always consider all three events and see what the venting requirements are versus what is available. If there are any manway emergency vents (commonly provided for fire relief) in addition to the typical pressure/vacuum vents, you could take credit for the man-way vents for 'an unusual' relief scenario (some companies only allow the 'normal' inbreathing, outbreathing requirements to be met by the pressure vacuum vents and credit for the man-way vents are only allowed for a fire case). If you don't have emergency man-way vents and have some spare man-ways available, you could always look at installing them. Protectoseal and Shand & Jurs are a couple of manufacturers I know of.
