The headline figure in IPCC Working Group III summary of a 3% reduction in global GDP in order to stabilise atmospheric concentrations at less than 450ppm does not mean that global GDP would be reduced by 3% every year. Rather, the estimate is 0.12% lower annual global average growth each year up to the year 2030, meaning global GDP would be 3% lower in that year than it would otherwise be (Table SPM.4 on page 14).
The summary further notes:
depending on the existing tax system and spending of the revenues, costs may be substantially lower under the assumption that revenues from carbon taxes of auctioned permits are used to promote low carbon technologies or reform of existing taxesand:
studies that assume the possibility that climate change policy induces enhanced technplogical change also give lower costs. However, this may require higher upfront investment (page 15)...In other words, the cost could be less than 0.12% of GDP per year, and we can afford not to put the planet at high risk of dangerous climate change.
...in all analysed world regions near term health co-benefits from reduced air pollution as a result of action to reduce GHG emissions can be subtantial and may offset a substantial fraction of mitigation cost (page 16).
[P.S. Climate curbs: Who will buy? by Paul Reynolds is good as brief analysis goes]