These are three very different sports it is hard for the body and feet to adapt to all three activities in one go to prevent blisters and other injuries?

The swim is first is usually done in a wetsuit, which is necessary but can also cause irritation and blisters, I would suggest 2 toms SportShield on those areas, which is an amazing product giving 100% 24 hour waterproof protection with one application, always just apply to the area that is giving you problems. Do not apply to the whole of the foot or broken skin

Races can be won or lost at Swimming/cycling/ running cross overs and most aim for them to be transitioned in the quickest time possible. A lack of preparation at this point can cause blisters later on at the bike and run stage.

Socks or no Socks? 

Some athletes wear them and some don’t, whatever you find works for you is absolutely fine. A little extra time in transition period, preparing your feet for the next stage can pay dividends in the last stage of the event. 

Moisture wicking Socks

Such as Darn Tough will naturally be the better choice to keep the feet blister free but socks are difficult to put on wet feet. Keep a microfibre towel at the transition stage before the socks go on, which will dry the feet as quick as possible. For those who don’t wear socks and still get blisters, use Engo patches on the hotspot areas of the cycling shoes and running shoes, so the friction has been reduced on those contact areas before the race starts. Use 2toms sports shield only apply on those areas that are prone to blistering as used liberally all over the foot could cause a loss of traction in the run stage

Blister prevention Skin taping

KT Pro- tape is 100% waterproof, synthetic tape, this would be my only recommendation in tapes and must be applied an hour before the swim, the skin can be prepared beforehand with alcohol wipes and an application of skin Tac to improve the staying power of the tape. 

Armaskin liner socks

would be a brilliant solution after the swim stage but more time is needed to put them on and that would be the trade off