In QGIS 2.18 there was plugin "Copy layers and groups to clipboard". It was allowing just in two clicks to copy/paste layer(s) with style and etc. from one project to another.
Is there something similar in QGIS 3.0?
You can drag and drop layers or groups between QGIS windows.
Note, this may not work reliably. As commented by QGIS developer ndawson - "it often results in a kind of "corrupted" layer, which has no features and can't be styled". Though I personally have not come across this issue.