Yes, the third coat of SLC primer is fine because the other stuff has long since dried and lost its effectiveness. You should verify this by reading the instructions for the specific brand of SLC that you bought.
As for mixing the SLC...two bags together with the correct amount of water will still require the same amount of time for mixing. One reason that I buy Laticrete SLC is that it has a pot life of 30 minutes. That lets us mix all the bags in multiple buckets, let them sit, give them one final mix with the drill, and then pour in on continuous shot.
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Kitchen & Bath Renovations (VA USA)