Apple is reportedly planning to sell the iPhone se and the iPad Air 3 starting March 18. This would apparently closely follow a reveal of both products at an event on March 15. This move would be atypical of Apple, which tends to favor at least a week of space in between a product announement and that products release.
Apple is currently planning to introduce a new 4-inch iPhone dubbed the "iPhone 5se" and a new iPad Air at an event on Tuesday, March 15th, then put the products up for sale online and in retail stores as early as Friday, March 18th, according to sources. Apple is unlikely to offer pre-orders for the new devices, according to sources who cautioned that the plans could still change.
The iPhone se is reported to be Apple's new 4-inch iPhone, and is said to take after the iPhone 5s, though with rounded edges like the iPhone 6 and 6s models. It will reportedly have an Apple A9 system-on-a-chip, along with NFC for Apple Pay support.
As for the iPad Air 3, it's said to take after the recently-released iPad Pro featuring a speaker near each of its four corners. The 9.7-inch tablet will also reportedly have a Smart Connector and Apple Pencil support. It's also said to have a flash for its rear-facing camera.
Keep in mind that nothing, including these devices and their potential release dates, is final until Apple announces them.