As a competetive player you should keep an eye on the FAQs.
"Q: What happens with a multi-unit attack, such as a
nova power, that triggers Strength from Death before
A: In the case of nova powers, templates or blasts,
resolve that attack completely before taking any
Following this entry I'd say once you are done with your attack. So if all three have Burst Cannons you do all of those before Soulburst comes into play but if you have a mix of Burst Cannons and Ion Cannons the Soulburst would squeeze inbetween once you are done with one of the weapons (only if it killed a unit of course).https://www.warhammer-community.com/2017/03/03/new-gathering-storm-ii-faq/