Interesting scenario. You have 48 hours to spread across three shifts but you don’t know the exact distribution until the task actually starts, and calculating from a planned start is probably impossible based on the dependencies of prior tasks. One question that I can only provide an assumption is whether you know the task is “exactly” 48 hours or is that an estimated amount. I’ll give you the easy answer. Assign the three shifts equally until the task starts. It’s a 48 hour task so you have over a week to make an adjustment, and if it’s not a known exact 48 hours then it will always be an estimate.