3: the aggregate of the underdeveloped nations of the world
One must also define the term underdeveloped. Both Haiti and Dominican Republic are underdeveloped.
The question is, then, which of the three definitions of Third World apply.
Dominican Republic fits more under definition 1. Dominican Republic is not aligned with non-Communist blocs...(i.e. NATO)
Arguably, Dominican Republic also fits under definition 3 as it is an underdeveloped country, but compared to its neighbor, Haiti it is more developed. It's other next door neighbor, Puerto Rico, is more developed.
To a Dominican I would assume DR is a developed country because their frame of reference is Haiti.
Just as 'developed' countries are not all equally developed, developing (under-developed/less-developed) countries are not all at the same stage of development...
There are various ways of categorising countries other than just developing/developed...for example the world bank categorises countries by income groups as follows:
"Economies are divided according to 2011 GNI per capita, calculated using the World Bank Atlas method. The groups are: low income, $1,025 or less; lower middle income, $1,026 - $4,035; upper middle income, $4,036 - $12,475; and high income, $12,476 or more.".
FWIW, per WB the DR is in the upper middle income group.
By per-capita GDP (PPP method) it is ranked about 87 (from the top) out of 183 countries, ahead of several Eastern European countries (Ukraine, Albania and others)...The DR's per capita GDP is about 8 times (800%) that of Haiti.