I don't think splitting the Metro order would be of benefit, it has to be 3 different retailers as per their terms. (those with the better card will have $10 purchases instead).
1. To qualify for this offer, you must make an eligible purchase of $20 or more at each of 3 different participating Partners using your BMO®† AIR MILES® Mastercard®* between September 1 and September 30, 2018, inclusive. Participating AIR MILES® Partners may be added or removed at any time without notice. Visit
https://bmoairmilesmastercard.airmiles.ca/en/home?vr=nfs3 for current participating Partner details. Eligible purchases of $20 or more must be made in a single transaction. Purchase transactions do not include cash advances, cash-like transactions, interest charges, fees, credit or debit adjustments. This offer is valid only for the AIR MILES Collector Number associated with your BMO AIR MILES Mastercard account. A limit of one (1) 500 Bonus Miles offer applies per Collector Number. To qualify for this offer, your BMO AIR MILES Mastercard account must be in good standing at the time the Bonus Miles are awarded. Offer is subject to change without notice.
2. BMO AIR MILES Mastercard cardholders will earn 2 AIR MILES Reward miles for every $20 in purchases at participating AIR MILES Partners. Participating AIR MILES Partners may be added or removed at any time without notice. Visit airmiles.ca/2xMiles for Partner details. Miles will be calculated on the total eligible cumulative purchases made at all participating AIR MILES Partners and credited to your Account within 90 days after the end of the applicable statement period. The number of Miles earned will be rounded down to the nearest whole number. Fractions of Miles will not be awarded. Eligible purchases are purchases charged to your credit card account less refunds and excludes cash advances, cash-like transactions, interest charges, fees and credit or debit adjustments. The “2x the Miles” benefit is applicable to the following BMO Credit Card products with no annual fee: BMO AIR MILES Mastercard, BMO IGA AIR MILES Mastercard, BMO Sobeys AIR MILES Mastercard, BMO Shell AIR MILES Mastercard, BMO Affinity AIR MILES Mastercard cards (excluding BMO SPC Affinity AIR MILES Mastercard cards and the BMO SPC AIR MILES Mastercard). This benefit does not apply to any BMO AIR MILES Mastercard not named in the list above.
3. Offer is cumulative over the statement period. Award of AIR MILES Reward miles is made for purchases charged to your account (less refunds and excluding cash advances, cash-like transactions, interest charges, fees, credit or debit adjustments) and is subject to the AIR MILES Reward Program Terms and Conditions and the BMO Mastercard Cardholder Agreement. The number of Miles will be rounded down to the nearest whole number. Fractions of Miles will not be awarded