It's extremely hard to predict your chances of getting a US visa but as an Indian, I can attest that it's quiet hard for the citizens in subcontinent to get a US visitor visa. It's probably because people in India, Bangladesh, Sri Lanka, etc. often overstay their welcome and end up being illegal immigrants. With the political climate in the US generally negative towards immigration, it's hard to speculate if you will get your visa or not.
While you do have some positives going for you(such as your trip in Europe), the recent rejection from the Japanese and the earlier rejection Australian embassy doesn't help. That being said, it's impossible for any of us here to even guess if you'll get your visa or not. The US visa process is an interview based process, a lot depends on your actual purpose of stay, your current income and most importantly how you actually answer the questions. A little bit of confidence can actually make a lot of change. But the only way you can find out is that if you apply. Do remember that it's generally harder for people in the "third world" to get a US visa. But there shouldn't be any harm in applying. Keep your fingers crossed and apply.