Have you been using your 5 gallon carboy for the extract batches? Partial (concentrated) boil and split boil are popular techniques for filling a fermenter that's larger than your kettle. Whatever method you may have been using to fill that 5 gallon carboy from smaller boil kettles could continue to work for your all-grain batches.
Your 13L kettle should be big enough to do a partial boil for the 3 gallon carboy. 10L is both a comfortable amount to boil in a 13L kettle and to ferment in a 3 gallon carboy with a reasonable amount of headspace. The boiled wort would only need to be diluted with enough water to restore what was lost to boiloff. You can even shortcut the dilution calculation which would normally be required. Set your pre-boil gravity to your desired Original Gravity, because they'll be the same after you've restored the water that boiled off.
If you want to target the 5 gallon fermenter with just one 13L kettle, I would recommend a partial mash technique. This is similar to what Roman was describing where you boil a small amount of high-gravity wort and then add sterilized water in the carboy. With partial mash, you can target a lower gravity from your mash and then bring it up to your intended boil gravity by adding a small amount of extract.
In a split boil, you'll have equal amounts of wort in two kettles. When it comes time to add hops or other boil additions, just put half in each kettle. A pair of 13L kettles would be just barely big enough to comfortably boil enough wort to fill your 5 gallon carboy, assuming you mean that the total volume of the carboy is 5 gallons and you're putting about 4.5 gallons (17L) of wort in it to leave some head space. If you don't have another 13L pot but you do have one that's at least 7L or so, you could do a split boil for that 3 gallon carboy. 5-6L in each pot pre-boil should boil down to a good amount of wort for that carboy.
If you want to do a split boil from all-grain, make sure that you run off your wort from the mash into a single larger vessel before splitting it or switch the run off back and forth between the two boil kettles so that they have a similar gravity.
All that said, if you're planning on targeting the 3 gallon carboy you should be able to find a no-frills 20qt or 24qt stainless steel stock pot for under $100 CAD. Then you can just do a full boil and not hassle with any of that extra work I described.