Hello i was wondering if i could upgrade to a 2nd assistant engineer by sailing on a boat that has 3,000 HP?
Likely yes, depending on the arrangement of the boat. According to the Marine Safety Manual, as long as the boat has a “walk in engineroom” it is considered large enough for your time to count as “assistant engineer” time. So, you will get a 2AE, but with a HP limitation of 5000 (3000 x 1.5 rounded to the next thousand).
When you upgrade, ask for a Limited Chief Engineer (Oceans). You should get it without further testing.