Good question Liza.
You are an advanced learner I assume? In which case I suspect you know that context is crucial? All three examples are synonyms in certain, limited circumstances. So you can use 'tantamount' to mean the same as 'equivalent' in some contexts, but not all. This is true of 'devise' and 'invent', and also 'obstacle' and 'barrier'.
Would you like to have a go at using these words in context, and native speakers (myself included) can comment on if you are using them correctly, or would you prefer it if I explained more first?