Just then three policemen come over ,they tell us the park is at 24 Renming Street.

我是英语老师，帮帮你。
Just then three policemen came over ,they told us the park is at 24 ,Renming Street.
just then 正在那时 属于一般过去时间，所以，come,tell用一般过去式。
the park is at 24 Renming Street 此句中的is 不要改成was，它表示客观存在的事实。
祝你进步！

Just then three policemen came over ,they told us the park is at 24 Renming Street.

Just then three policemen come over ,they tell us that the park is at 24 Renming Street.

come-came
is-was

"Just then"是“就在那时”的意思，时间状语，表明句子已发生在过去，而整个句子用的是一般现在时，显然不对。