В таблицу topic добавить строку event_id
В таблицу speaker добавить строку topic_id
В модель event

public function topic()
    {
        return $this->hasMany(Topic::class, 'event_id', 'event_id');
    }

В модель topic

public function event(): BelongsTo
    {
        return $this->belongsTo(Event::class, 'event_id', 'event_id');
    }
    public function speaker()
    {
        return $this->hasMany(Speaker::class, 'topic_id', 'topic_id');
    }

Speaker модель

public function topic(): BelongsTo
    {
        return $this->belongsTo(Topic::class, 'topic_id', 'topic_id');
    }

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