say I own the same domain name in .com, .org and .net
If I host a website at the .com and redirect the .net and .org to the .com. These would be 301 redirects with no actual web pages online for the .org and .net.
If I then have backlinks to the .net and .org will these count towards the number of backlinks for the .com web domain?
You would (presumably) only have backlinks to the .net and .org versions if at some time they had had sites with content of their own. But yes, if these are correctly redirected, then the link should count as a link to the .com domain.
When you redirect the .net and .org to the .com, the backlinks from the .net and .org will also be counted towards the .com. It works like, as soon as you click or go to the .net site the url itself will redirect itself, it does not matter where the link for the .net or .org is coming from, the url will automatically get redirected when the redirected website is called.