Instagram has reached 3 billion monthly users, cementing the network as one of the most popular consumer apps of all time and leading parent Meta Platforms Inc. to put even more emphasis on the tools that keep driving growth: short-form video and private messaging.The app is changing its home screen navigation bar to highlight private messaging and Reels, Instagram’s video rival to ByteDance Ltd.’s TikTok, which will make those features easier for users to find. It’s also running a test in India...