Gnucash category structure for funds distributed across multiple bank accounts

GnuCash allows hierarchical listing of accounts, with the parent also summarizing the children when in the accounts' list. So you can create "TFSA Accounts" account under "Assets", and create separate account for each of the TFSA's under that "TFSA Accounts" parent account. You can mark it as a placeholder for your convenience so that you won't assign transactions to the parent by mistake.

When you're looking at the "Accounts" page, you'll see something like this:

-Assets ................................. $115010.50
  -Savings Accounts ..................... $115000.00
     -Some Non-TFSA Savings Account ..... $100000.00
     -TFSA Accounts...................... $15000.00
        -TFSA Account 1 ................. $8000.00
        -TFSA Account 2 ................. $2000.00
        -TFSA Account 3 ................. $5000.00
  -Checking Account ..................... $10.50

When you collapse the "TFSA Accounts" it will look like this:

-Assets ................................ $115010.50
  -Savings Accounts .................... $115000.00
     -Some Non-TFSA Savings Account .... $100000.00
     +TFSA Accounts..................... $15000.00
  -Checking Account .................... $10.50

And you'll be able to see exactly how much you have in all of them, without trying to summarize them every day.

Transactions go as usual, for each of the accounts (you'll have to have a withdrawal transaction from each, when you pay the $15000 to the roofer, anyway).

Your question is a little ...unusual... in that you want GnuCash to show where you might, potentially, spend money at some unspecified time in the future. GnuCash's core intention is to show where your money currently is, or the account categories of your past actual income and expenditure. That's not to say that GnuCash can't do what you want it to - we just need to be a bit more creative.

You can start by creating some Asset accounts to show your real-world bank accounts and their notional sub-accounts, like this:

Note that there is a HouseReno sub-account under 3 different real-world accounts, because that's how you want to spread your money. For simplicity, I've only put notional accounts for Vacation, Bikes, and Other under the Savings account - but you can create those sub-accounts under the TFSA accounts as well, if that would be useful.

Create transactions to show the actual money in each real-world account, distributed as desired across the notional sub-accounts.

Now, go to the GnuCash Reports menu and select Transaction Report. With the report window open, configure the Options for the report. In the Options/Accounts tab, you will cunningly select ONLY those accounts that include HouseReno in the account name. Exclude all other accounts from the report. In the Options/General tab, set the Start Date and End Date to be a wide range, so as to capture all past and future transactions in those accounts.

SAVE the report configuration with a meaningful name, like "HouseReno Balance".

Next week, or next year, with only 3 mouse clicks you will be able to get a report that looks like this (with updated balance, of course):

Job done!