I'm stuck on Mission 3, where you have to defend X number of silos.

I looked at some walkthrough videos and noticed that they only have 2 enemy ships attacking the silos whereas I have 6 of them. Here's one of those walkthrough videos.

My ships stand no chance of beating 6 enemy frigates, no matter what strategy I use.
I can't see anything in the gameplay settings that would allow for a difficulty adjustment.

Does the game adjust to how many ships you have ? which means I would actually be better off in the game having fewer ships ?!

I think you've posted this in the wrong forum, try here.

As it is you're doing better than I am, I'm still on the second mission. In answer to your question: I have no idea - but someone else must know.

It's the Remaster, the original keeps crashing on me, which is funny because I thought it would be the other way around.

I've answered my own question: I loaded a previous save from the end of Mission 2 and scrapped most of my fleet, to only end up with 15 or so in total. The result was that I only got 2 enemy frigates in Mission 4, not 6.

This was much more manageable, and I beat the mission on the first attempt. It was quite easy.

So 'yes' the game definitely adjusts the AI size to your fleet size.

It was confusing, because I read how some people found Homeworld to be very easy while others found it to be very difficult. This explains it, if it's like this throughout the game. Oops, apologies, I didn't think of that. I'll remember that next time.