It's true that one must look at the portfolio as a whole, and think you're fine as long as you trades fill within the span of an hour. I always place limit orders, and start with the least liquid one in a group (whichever has the widest bid/ask spread, likely CGL.C) since this will be the most difficult one to fill.
What I'd do (edited, revised)
1. place one limit buy order on each ETF
2. wait 15 mins (or longer I suppose) but consider this a hard deadline
3. if something hasn't filled, modify the order and meet the asking price
In step 1, choosing a good limit price, with a high probability of being filled, is a bit of an art. For CGL.C and MNT, I would probably place my bid at the mid point between the bid & ask (half way). XIU is liquid and very active, so I might bid one or two cents below the current asking price. Bond ETFs have low volume and don't move much. I wouldn't get more ambitious than one cent below the asking price on a normal day.
After the 15 minutes are up, I would revisit any unfilled orders and change them so they are guaranteed to fill. At that point I would do a limit buy at the asking price or perhaps one or two cents
above the asking price. There is no risk of overpaying when you do this, since you'll still get the lowest ask for the number of shares you want. Note that if you're buying a lot of shares, it may take more than the current asking price to fill all shares.
That's effectively a market order, but it's always safer to place limit orders.
In step 3, I think it's important to give up on trying to get bargains, and just buy. This prevents you from falling into the trap of "chasing the market" as prices move against you. My view is that you already took a stab at getting a bargain (step 1) and now it's time to complete the trade.
Aside 1:
here's that portfolio in the Visualizer using ZFS for the cash component. That picture is nice because it starts in a bad year (where gold & long bonds fell hard) so you're getting a pessimistic view.
Aside 2: I'll backtrack on something I said earlier, and I think it's fine to have foreign diversification in the stocks. If you look at my own RRSP portfolio in #302 you'll see that I mix XIU & ZSP