```python
from typing import List, Dict

def dose_totals(log: List[List[str]]) -> Dict[str, str]:
    reagent_totals = {}

    for reagent, amount in log:
        if not reagent_digits := re.match(r'[-+]?\d+(\.\d{1,3})?', amount):
            raise ValueError(f"Invalid amount format: {amount}")

        amount_float = float(amount)
        if reagent in reagent_totals:
            reagent_totals[reagent] += amount_float
        else:
            reagent_totals[reagent] = amount_float

    result = {}
    for reagent, total in reagent_totals.items():
        total_str = f"{total:.3f}"
        result[reagent] = total_str.rstrip('0').rstrip('.') if '.' in total_str else total_str.lstrip('+')

    return result
```