The money in the traditional IRA can only be used to satisfy the RMD of your traditional IRA(s). It cannot be used to satisfy the RMD from a 403(b). If you want to avoid having to take RMDs from your 403(b) in 2019 and beyond, after satisfying your 2018 RMD from the 403(b) you'll need to roll the remaining balance in your 403(b) over to an IRA in 2018.
Regarding aggregation of RMDs, only traditional IRA RMDs can be aggregated with those of other traditional IRAs and taken from a single traditional IRA, and 403(b) RMDs can be aggregated with those of other 403(b)s and taken from a single 403(b). RMDs for other types of qualified retirement plans must be satisfied with a distribution from the particular plan.
If you have other accounts subject to RMDs and are not IRAs, the RMDs for those accounts will have to be satisfied from those accounts; the RMDs from those accounts cannot be satisfied with a distribution from an IRA. After satisfying the RMD for those qualified retirement plans for the year you can roll the remainder over to an IRA.
https://www.irs.gov/retirement-plans/retirement-plans-faqs-regarding-required-minimum-distributions#...
