Please note that these are for url links, not embed codes.
X (formerly known as Twitter): Log in to Twitter and go to the profile or feed the tweet is in and then select the tweet to expand it in the browser window. Or, select a tweet from your feed you've encountered. From there, it will have it's own url you can copy and paste (this is what that looked like before the rebrand, but it is the same principle):
Instagram: Log in to Instagram and go to the profile you want a post from or select a post from the main feed:
Facebook: Log in to Facebook and go to the profile/group/page with the post you want to link to or select a post from your feed. If it's a single image post, you can just grab from the url bar. But if it has more than one image, you should go to the post's three dots > embed > advanced settings > and then you can get the url of the post under the header "Code Generator."
View the box and links below for more information on how to use social media: