/ What knot?
Thumb knot, figure 8 or clove hitch? The first two can both roll off, but cannot pass the carabiner or person attached to it so are safe, but will then suffer the same problem with the clove hitch. The latter will keep the second exactly where the rope runs, so if the bottom person falls off the second will be pulled to where the rope straightens to.
A double thumb knot will work to prevent rolling and leave a tail so the second has a bit of freedom. Alpine butterfly is still the most reliable (I think, as it doesn't roll and close the loop,) so would suggest that.
The stresses on your harness would be messy depending on the situations. If the bottom person falls the knot can become a slipknot, so you would end on top of the last person (If you haven't melted your harness loops through.) You can avoid this by larks footing onto a carabiner and clipping that to the harness (I assumed that you meant larks footing directly onto a harness...)
Measure appropriate distance between climbers.
Take approximately a 1metre bight (rope dependent) and tie overhand at point measured out.
Now tie 2nd person in as follows:
Tie an overhand on the bight about 30cm from the first overhand to provide some leeway for 2nd climber.
Thread bight around harness as normal.
Feed bight through the overhand knot.
Tie a single stopper knot.
Clip remaining small loop back to the tie-in loop you have formed with a small screwgate.
The MLTUK Handbook - Rock Climbing, Essential Skills and Techniques by Libby Peters (page 187 of 1st Ed.) describes a very similar method but re-threads the overhand rather than passing the bight through and tying the single stopper knot.
Either method works but with the variation I described it is much easier and quicker to get a well-dressed knot in a thick single rope.
Hope that makes sense.
Or if you have one rope with someone in the middle, alpine butterfly and a screwgate (or two if you're that pedantic).
Re-threaded overhand knot directly through the loops on your harness the belay loop passes through, with the bite clipped through a locking krab, or an alpine flutterbye attached by a locking crab to your belay loop are my two suggestions, both because they're simple, and easy to tie, and being a bit slow, I'm all for idiotproof ideas!
A lot of people seem to have issues tying an alpine butterfly using the "twisted loops" method, but if you use the method over the palm of your hand, it's pretty obvious and easy to follow.
This is what I would do. Standard practice in the alps and is what about six guides have done on various courses I've been on.
I would rethread the overhand though rather than the stopper knot and reduce the 30cm isolation loop if the bloke in the middle was a liability. Also, keep the two seconds close to each other unless moving together.
One climber on end.
About 5 metres up do a figure of 8 on the bight with a long head to create the isolation loop.
Got the third climber to tie in with rethreaded overhand and clipping the tail back to the live rope.
For the isolation loop I was concerned about it rolling back towards the climber on the end of the rope so thats why I used the figure of 8. I am happy to be proven paranoid about that?
the main concern is that you have your seconds isolated from one another with a nice big bight of rope. I'd use an alpine butterfly or an overhand for that and let them tie in with a mega knot backed up with a screw gate or the fig 8 backed up larks foot.
Id also think about having the "middle man" not in the middle more like 10 m above the last man and taking a good number of coils in, so I could get back down to the seconds if needed. Even better you tie into the middle, or better still have them on two halves.
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