What is the impact of studying a third language in a second language?

when learning a third language, what are the relative advantages or disadvantages of studying the third language in either my native language or my second language?

More specifically, if I choose to learn a third language in my L2, how will this going to aid and/or harm my learning or that third language? How will it going to aid and/or harm my L2 learning? Are there any studies on this?

For example: My native language is English and my L2 (Swedish) is at about a B1 level. Lets say I'm interested in learning Russian and I have found both an English-->Russian course and a Swedish-->Russian course online. What are the advantages and disadvantages of me taking each of these courses?

Generally it's easier to learn new languages from your native language, whether you're learning L2, L3, L4, etc.

Learning L3 using L1

You will understand things better when they are explained to you. If you try to learn tricky grammar rules and the definitions of obscure vocabulary you will likely not understand the explanations as well in L2.

Your native vocabulary is almost certainly better than that of a second language. It's hard to learn L3 vocabulary using L2 if you don't know the same vocabulary in L2. In fact you may subconsciously revert to L1 to understand it.

If L1 and L3 are similar, you'll be able to take advantage of cognates and other similarities more than L2 and L3 because you'll be less likely to confuse your native language with another.

Learning L3 using L2

However, if you would like to challenge yourself, there are benefits to learning L3 using L2, such as the simple fact that you'll strengthen both languages whereas if you learn using L1 your L1 proficiency probably won't be improved much. However, this multi-tasking will slow your progress in L3 compared to learning with L1.

If L2 and L3 are related, you'll be able to see the similarities between them perhaps more easily than if you're learning from L1. However, this may be problematic as you may also confuse similar words or grammar constructs (more likely to happen than with L1 and L3).

Most of this I know simply by experience. Most prominently I've learned Portuguese (L3) using both Spanish (L2) and English (L1); more easily with the latter. I'm also learning German (L4) using English (L1) and it's a struggle. I can't imagine learning German using Spanish or Portuguese.