What is the rule for a series of third tones?
It might be subtle, the series of tones would be 2-半三声-2-3. (半三声 means only the first half of the 3rd tone, without the rising tone part.)

Firstly 我也想买 could be separated as 我(也(想买)), and according to the rule about the 3rd tone sandhi, rule #4.1 (for 也(想买)) and #2 (for 我(...)) match and then leads to 2-半三声-2-3.