I don't have specific experience in the issue of tandem (triandem?) nursing,
but do have some experience with cold sores in the breastfeeding dyad.  Cold
sores are caused by the herpes virus, and are contagious, being spread by
contact.  
My younger son acquired a cold sore at daycare when he was 11 mos.  I
thought nothing of it and breastfed as usual, and unfortunately acquired
several blisters on the left breast.  This was unbelievably painful for both
of us - especially since it was a primary outbreak.
Cold sores are spread by contact, so avoiding contact minimizes spread. 
Treating with an anti viral cream (Zovirax or Denavir) may speed healing and
minimize the chance of spread.  If the lesion is in an area that can be
covered, covering with an occlusive dressing also minimizes spread.  Herpes
virus is not known to be transmissable by breastmilk, so the risk is not
really in breastfeeding, but in contact.  
I would think you could minimize risk to the non-affected nursling by
washing the breast between nurslings, or limiting the affected nursling to
one breast and the non-affected to the other.  In the case of three
nurslings, you could perhaps encourage the 3 yr old who is currently
affected and the 5 yr old who is not (but has been exposed himself in the
past) to use the same side and reserve the other side for the infant. 
Unfortunately, herpes is known to be transmissable even when there is no
current outbreak, so there is no way that I know of to completely eliminate
the risk of transmission.  (Either this info may help you feel better about
continuing to breastfeed during an outbreak, or scare you to death, I'm not
sure which!)
I'm not sure of any good way to avoid contracting it yourself, but if you
do, the same anti-viral creams work, or you could take systemic anti-viral
meds to speed healing.  I found the outbreak on the breast to be very
painful, and also had enlarged lymph nodes under my arm.  My beloved FP had
recently moved away and I could not convince the new doc to give me meds so
suffered through for close to 2 weeks.  I would hope if you found yourself
in a similar situation you would have better luck!  We were able to
persevere through this episode and get back to painfree breastfeeding
eventually.

Jennifer Tieman
Family Physician
Mom to 4, including my nursling Caroline Rose (age 32 mos.)

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