The value of a Miroku model BC3000 12 gauge under and over shotgun would depend on a couple different factors. Most important of these factors would be the condition.
$1
Depending upon condition, $400 to $650
Miroku values vary widely based on what model and when the gun was produced. The Miroku factory has produced guns for Browning, Winchester, Remington, and Charles Daly to name a few. You need to give more specific information to get a better answer.
You will have to contact Miroku to find out.
6000 euro
Should bring $500.
You will have to contact Miroku
depending on condition around $400 to $650 used.
300 USD or so
The Miroku shotguns will typically bring 80% of the value of a Browning Citori in similar condition.
Miroku guns made under their name does not have serial info published. Winchester, Charles Daly and Browning have all been made by Miroku in the past. Do you have a model?