It is practically not possible. There are a lot of complications that govern an economic union especially in these three countries. Firstly, Turkey is a NATO country aspiring to be a part of EU, both of which have imposed severe sanctions on Iran and despite the latest thaw, continue to keep a tight chain around Iranians. This means that Turks even for economic reasons cannot be involved in full force with the Iranians even if they could do the same with you because their obligations towards their alliances and the pressure from other countries might push them to not open up to the full potential.
Secondly, Iranians on their part are extremely independent. They enjoy taking the role of leadership and do not like being dictated into doing something whether it is financial or economic or military; their fierce independence in these matters might pose a problem.
Thirdly, the disparity factor comes into play. Turkey is the most industrialized country and also the richest among the grouping. Its economic power comes from its massive trade with the EU and the Middle East, heavy focus on industrialization, free market economy, ease of doing business and a culture that is highly open to innovation, invention and different thought. It is also politically a highly trusted entity around the world and is looked at as a reliable partner in terms of economic and trade factors.
Though Iran is also a reasonably prosperous country with oil wealth and a desire to be strongly industrialized, its limiting factor is its opaque business competitiveness policy, its industrial capability is little known outside Iran or perhaps the Middle East, Its exports are regionally limited apart from oil and it has every single industrial powerhouse country (except China and Russia) breathing down on its neck. Pakistan, in contrast, is the least developed in the grouping, suffering from acute financial, political and national security related problems. Terrorism, imbalance in the government-military equation, very little focus on industrial development of the country, no indigenous production of strategic commercial heavy platforms, severe power woes, limited export industry other than agriculture and textiles... these are some of the key problems.
Pakistan's GCI ranking is much better than our own country, but we are talking about an economic union that involves in highly diverse countries with varying geopolitical problems.
Compare this to the Eurasian Union which mainly involves in Russian speaking countries either officially or as native languages. These countries have the same or at least similar economic models, business investment approach, currency ratings, education base, linguistic and cultural base to a large extent, all being ex-Soviet or Soviet-allied states of the yore. Despite corruption, there is no threat of terrorism, mass riots and attacks from anywhere, which allows a strong possibility of a dominating common and stable currency; most likely named something like the Eurasian Ruble.
A Turkey-Iran-Pakistan economic union maybe not possible in this decade at least.