You should include an independent EHG and CHG-rich source to have a better picture of the genetic history of the region. Northeastern Europeans, including Slavs, have a lot of extra EHG in addition to the EHG in their steppe admixture, and there was CHG-rich movement into the Balkans and Italy since the Late Neolithic probably via Chalcolithic and Bronze Age Anatolia. So, using only 3 reference populations will skew results a lot in favor of Yamnaya, which had EHG and CHG.
Again I also stress that I have noticed, comparing the results of Vahaduo to those I get using the R software and nMonte2, the Multiple test does not work as well as the single one, the results are somewhat distorted and less accurate, so I'd caution against reading too much into them. Testing each sample separely using the Single tool is more reliable.