1. There will be more than three hundred scientists attending the meeting.

现在分词 attending the meeting 并没有发生，是因为 There be 中有 will。

如果用不定式也可以，就把 There will be 改为 There are，同样也表示未发生。

2. There are more than three hundred scientists to attend the meeting.

如果不定式用进行体，跟“将来进行时” 意思一样。指“在将来某个具体时间里正在进行的动作”。

3. There will be more than three hundred scientists to be attending the meeting.

以上三句都表示“将要参加会议”。