"Since 1985, they opened 3 provinces to foreign investments. In 1990, they further opened some cities situated in the coastline region."

By "since 1985" did they mean from 1985 to 1990? Is that even correct grammatically? Shouldn't we say "from 1985 to 1990" instead?
I'm honestly not clear of the author's intention. I would have used present perfect: Since 1985, they've opened...

Usually, if you read "Since [date]," your assumption would be from that date until the present. If the author wished to specify the period 1985 through 1990, then your rewrite (and the use of the simple past) is correct.
"Since 1985, they opened 3 provinces to foreign investments. In 1990, they further opened some cities situated in the coastline region."

In 1985 they opened 3 provinces to ............. - all 3 provinces were opened in 1985

Since 1985 they have opened 3 provinces to ...... - between 1985 and now 3 provinces have been opened.

Between 1985 and 1990 they opened 3 provinces to ....... - the provinces were opened sometime during the period 1985-1990
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Maybe I should have quoted this instead:

When it decided to reform the national economic setup in 1978, the Chinese government embarked on a policy of opening to the outside world in a planned way and step by step. Since 1980, China has established special economic zones in Shenzhen, Zhuhai and Shantou in Guangdong Province and Xiamen in Fujian Province, and designated the entire province of Hainan a special economic zone. In 1984, China further opened 14 coastal cities—Dalian, Qinhuangdao, Tianjin, Yantai, Qingdao, Lianyungang, Nantong, Shanghai, Ningbo, Wenzhou, Fuzhou, Guangzhou, Zhanjiang and Beihai—to overseas investment. Then, beginning in 1985, the state decided to expand the open coastal areas, extending the open economic zones of the Yangtze River Delta, Pearl River Delta, Xiamen-Zhangzhou-Quanzhou Triangle in south Fujian, Shandong Peninsula, Liaodong Peninsula, Hebei and Guangxi into an open coastal belt.In 1990, the Chinese government decided to open the Pudong New Zone in Shanghai to overseas investment, and opened more cities in the Yangtze River valley. In this way, a chain of open cities extending up the Yangtze River valley, with Shanghai’s Pudong as the “dragon head,” has been formed. Since 1992, the State Council has opened a number of border cities, and in addition, opened all the capital cities of inland provinces and autonomous regions. In addition, 15 free trade zones, 32 state-level economic and technological development zones, and 53 new- and high-tech industrial development zones have been established in large and medium-sized cities. As a result, a multi-level, multi-channel, omni-directional and diversified pattern of opening, integrating coastal areas with riverine, border and inland areas has been formed in China. As these open areas adopt different preferential policies, they play the dual roles of “Windows” in developing the foreign-oriented economy, generating foreign exchanges through exporting products and importing advanced technologies and of “radiators” in accelerating inland economic development.