The only we need is skill
Nobody said skills were the
only thing you need, but their relative importance certainly eclipses the other things you asked about, because without them one gets nowhere.
very interesting: starting capital is irrelevant, strategy is irrelevant, instrument is irrelevant...
Well, let's take a look at those assertions. My guess is that you're dismissing them from a sarcastic outlook, but that might be unwise because a couple of them are actually pretty valid, overall ...
"Instrument": there are people making steady monthly incomes from trading a huge variety of different instruments, each according to their own skills, knowedge and experience, so in that sense it isn't really highly relevant to what you wanted to know. Your own circumstances and starting capital can make it relevant to some extent, though, it's true: for example, if you're underfunded to trade futures (a sensible ball-park starting figure for that might be in the region of $25k, after you've developed all the appropriate experience and skills), you might want to exclude those from your consideration?
"Strategy": again, there are people making steady monthly incomes from trading with a huge variety of different strategies, each according to their own skills, knowledge, availability, interests and experience, so in that sense it isn't really highly relevant to what you wanted to know. Your own situation, education, interests, background and financial orientation can make it relevant, of course, but you told us
absolutely nothing about any of them, so that makes it hard for us to advise you on that front, too: we'd really be guessing, more or less randomly. And I think you probably wanted more helpful answers than random guesses?
"Starting capital": ok, this is obviously relevant, but still hugely variable ... one way of looking at it would be to start from the assumption that you're gradually going to become one of the most successful 1% of retail traders. It's a hell of an assumption, of course, but for the sake of offering a more concrete answer to one of your questions, let's assume that, for the moment? In my estimation (and I can't prove this, and neither can anyone else), fewer than 1% of retail traders, eventually, after years of education and skill-acquisition and thousands of hours of screen-time practice after mastering all the basics, eventually earns a steady-ish 5% monthly return on their capital. The $3k/month average you're asking about is 5% of $60k, so that would put your starting capital for that kind of average monthly income at about $60,000. It's vague, it assumes a lot, and it's open to discussion, but it's a ball-park answer,
if you're (eventually) among the most successful 1% of retail traders.
Personally, I think you'd be well advised to be thinking about
how to become among the most successful 1% of retail traders.
If it helps/interests you at all, there's a description of how I started off in
this post.
I hope you find that a more practical and helpful reply to you than my response above.