the 3m limit was from memory an old railtrack requirement it may be fallout from this.
It related to the tube length as opposed to its weight.
It's no longer about how big and tough we are as scaffolders more about protecting ours and others longevity, Bags of cement were 50kg when I was slinging them in the 60's but things have moved on.
SG6:15 states, "an individuals age, strength, level of skill and experience will affect how much a person can safely handle"
Section 4.2 of the same document still refers to hemping 21ft tube (all be it contentious) thus must be acceptable to the industry.
None of which is in question or in play here, the issue is the clients spec requires the longest tube to be 3m this is your limiting factor not what you can lift.
They have even asked you to price accordingly, the easy option would be to price the job with hired system, charged at the rate you pay +20% and make your money on the labour.