Are there any Mandarin dialects with three tones? 有只有三个声调的官话方言吗？东干语有四个音素的声调，但是在没有变调的上下文只有三个不同的声调, 例如在短语的末尾。有没有在每个上下文都用三个声调的方言？
Are there any Mandarin dialects with three tones? Standard Dungan has four phonemic tones, but only has three in contexts where tone sandhi doesn't apply, such as phrase-finally. Are there any with three tones in all contexts?
Excerpt from Wikipedia article on Dungan

There are two main dialects, one with 4 tones and the other, considered standard, with 3 tones in the final position in phonetic words and 4 tones in the nonfinal position.
Lanyin Mandarin

特征
兰银官话古入声清音声母和次浊声母字今读去声，全浊声母今读阳平。其中永登、皋兰、古浪、天祝四个地点平声不分阴阳，只有平声、上声、去声三个调；其余地点阳平与上声同调，只有阴平、上声、去声三个调。

Ürümqi Mandarin (Included in the above Lanyin topolect)

乌鲁木齐话有3个单字调：
阴平 ˦˦ 44
阳平 ˥˩ 51
去声 ˨˩˧ 213

The following might not strictly be Mandarin, but are interesting nonetheless.
Tianshui Topolect (Gansu)

There are only 3 citation tones in Tianshui dialect.

Xunhua Topolect (Qinghai):

循化话有3个单字调，湟中话有2个单字调。

(The Huangzhong topolect only has two tones.)
Nanjiang Topolect

兰银官话北疆片、中原官话南疆片基本上都是3个单字调，但调类不同。