If they really are 'heavy' they may not need feeding ... three boxes overwintered ? I would think, if the person who sold you them was any sort of a Warre beekeeper, that they would have enough stores to see them through. You are in an area where spring usually comes quite early so they will already be foraging - I have a microclimate where I live and mine are very busy.
As has been asked - are they Warres with just top bars or have you got frames .?
A lot of warre hives have viewing windows so you can look at the comb without disturbing the bees .. has yours got them ? If so, you can look in and see if the combs have capped stores or the cells are empty - without being able to see the combs you are working blind and can only judge by the weight .. if you don't know what 'heavy' really means in terms of what is in the boxes you are in a slightly awkward situation.
I assume the boxes were delivered to you without being split or opened up so you won't really know what is in there - one of the principles of Warre hives is that they are not intensively inspected - if they are top bar Warres it may be a bit awkward for a new beekeeper and frankly, without looking inside the hive you are not going to learn a lot about keeping bees. Inspections in Warres are usually just for space needed and queen cells and that's normally just achieved by tipping the boxes and looking at the combs from the bottom of the box.
Is the arrangement at the top of the hive a canvas quilt with insulation above ? If it is and you need to feed them you will have to have some sort of arrangement to get the Candipoline Fondant in contact with the bees - another box on top of the top box (as suggested) is an option. Or you could make an Eke with a lid and a feeder hole in the bottom and put the fondant in there, locating it between the quilt and the top box.
You are in at the deep end a bit I'm afraid. It's very early to be looking inside but if you are worried that they may not have enough stores then you have two choices - pick a nice warm day when they are flying well and have a peek in the top box to see what stores there are or just put the fondant on in a feeder of some sort and check later on in March.
You need to be careful feeding at this time of the year as you may find that they store it and the queen runs out of space to lay and the stored fondant ends up in your honey ...
There's a lot to learn and the techniques of beekeeping in Warre's are a bit different to more conventional hives.
There's a few Warre sites around on the internet:
https://beebuilt.com/pages/warre-hives
https://warre.biobees.com/
https://www.thewarrestore.com/warre-hive-management
There are more but some time spent reading the above will give you some idea of what you have got into.
As for time of day of feeding ... feeding bees is not like feeding a dog ... you put the food in the hive and leave it there - they will usually take it down and process it. Time of day is irrelevant except for the convenience of the beekeeper. What you must NOT do is put the stuff in a dish and feed outside of the hive ... you risk introducing disease from other colonies and the possibility that other colonies will be attracted to your apiary and may try to rob your colony.
So much to know ... and a short time to learn.