In terminal, if you do not specify a directory, the commands are searched for based on the PATH variable. To see what directories are being searched, in terminal type “echo $PATH”. If the ST executable is not located in one of those directories, typing in a command in terminal will not execute any command. That aside, you’ve specified two different paths. “~/opt/” is equivalent to “/home//opt” where is your username. Where as “/opt/sublime_text_3” is the specified path. That aside, I’m guessing the blog post said to symlink the executable to some location. Then add that location to the $PATH variable if it is not there already. The command to create a symlink is “ln”. You can google about symlinking. However, as it appears you are learning more about Linux, I’d say it might be worthwhile to look at the man page. This will provide you with documentation on the command. Of course, the same information is available online, but being able to read and navigate man pages is useful (in my opinion). To view the man page, in terminal, simply enter "man ", where is the command you are interested in. In this case, it would be “ln”.
If you would like more help, I’d also recommend adding a bit more information to your post. Some relevant information may be the steps you followed to install. Perhaps the commands you had run to set everything up if you remember. The blog you are referencing.