Concatenating pandas dataframes of different lengths

Question

I have several empty dataframes, all of different lengths, they all share some common fields, but each dataframe has some unique columns as well. In trying to concatenate them I keep getting a ValueError: Plan shapes are not aligned.

cols1 = ['Test', 'Col1', 'Col2']
cols2 = ['Test', 'Col1', 'Col2', 'Date']
cols3 = ['Test', 'Col1', 'Col2', 'Time']

df1 = pd.DataFrame(columns=cols1)
df2 = pd.DataFrame(columns=cols2)
df3 = pd.DataFrame(columns=cols3)

df_dict = {'project': [df1, df2, df3]}
for table, dfs in df_dict.items():
    result = pd.concat(dfs)

Ideally my output should have a columns list that looks like this:

['Test1', 'Col1', 'Col2', 'Date', 'Time']

Is there any way to get past this without using Merge. I don't want to use Merge because I won't always know the columns that are shared, etc...