In today's often fully booked and oversold flights, I would not count on a middle seat remaining open. They will also not block a middle if flying MCE if they are sold out.
My advice - buy the 3rd seat and bring an approved car seat to put into that middle seat.

Gold entitles all parties traveling with the Gold member to one free checked bag.
You are entitled to a free checked stroller and car seat which would not count against your baggage allowance. If you take a stroller to the gate, it will be gate checked. You can bring the car seat on if you have a seat for the baby reserved to use it on.

https://www.aa.com/i18n/travel-info/...g-children.jspI


Strollers

• Each ticketed customer is allowed 1 stroller and 1 car seat to be checked free of charge
• Strollers over 20 lbs / 9 kgs must be checked at the ticket counter
• All other strollers should be checked at the gate before boarding
• If you have a stroller and a car seat only 1 can be checked at the gate

Additional accessories

• Breast pumps and a small, soft-sided cooler of breast milk don’t count as your carry-on or personal item
• Play pens, wagons, cribs and 'pack and play' count as regular bags
• If any of these exceed carry-on limits they’ll be checked and incur regular fees