Dude, Third world was considered offensive nearly two decades ago when I was in high school. Where've you been? If you must refer to other nations as being worse off than ours, and it's best to not use "blanket" statements, then developing nation, LIC or or "Global South" is better. None of those terms is perfect however, and given that most of the reason many of these countries are in a shitty state is because we colonised them and are still basically enslaving people to mine our diamonds and grow our coffee and bananas and chocolate, you could just avoid referring to such countries in such a derisive and dismissive manner at all. Especially when many actually are having such a boom that they are likely to be more wealthy than us in the future.